NCERT Solutions Class 8 Science Chapter 10 Light Mirrors and Lenses

F R E E S T U D Y M AT E R I A L F O R E V E R Y S T U D E N T

CLASS 8 · SCIENCE

NCERT Solutions

Chapter 10: Light: Mirrors and
Lenses

NCERT Textbook — Curiosity

BOOK PAGES SECTIONS QUESTIONS MEDIUM

152 – 169 26 61 English

Solutions, notes, sample papers & more at 63 pages

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

CLASS 8 · SCIENCE · CURIOSITY

NCERT Solutions — Chapter 10: Light: Mirrors and
Lenses
A plane mirror always hands back an image the same size as the object. Curve that reflecting surface — or
shape a piece of glass with curved faces — and the picture changes completely. This chapter builds the ray
model of reflection, states the two laws of reflection, and uses them to explain why a concave mirror can
enlarge your face or turn it upside down, why a convex mirror always shrinks the traffic behind you, and why
a convex lens can set paper alight.

TEXTBOOK BOOK PAGES

Curiosity (Class 8) 152 – 169

SECTIONS QUESTIONS

26 61

MEDIUM

English

Probe and ponder — Page 152
Chapter opener

PROBE AND PONDER

Q1 Can we make mirrors which can give enlarged or diminished images?

Yes. We only have to bend the reflecting surface. A plane mirror sends every ray back without
changing how much the beam is spreading, so the image is always exactly the size of the object.
A spherical mirror — a mirror shaped like a part of a hollow sphere — is different, because its
surface faces a slightly different direction at every point.

A concave mirror (surface hollowed inwards) pulls the reflected rays together. Held close to
your face it gives an enlarged, erect image; taken farther away the image turns inverted
and then shrinks.
A convex mirror (surface bulging outwards) spreads the reflected rays apart, so its image is
always erect and diminished.

Page 1 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

Reflecting side Reflecting side

Concave mirror Convex mirror
Seen edge-on, the difference is obvious: the shiny side of a concave mirror is hollowed inwards; the
shiny side of a convex mirror bulges outwards. The shaded part is the non-reflecting back.

Why it happens: reflection at every point still obeys the same law — the angle of
reflection equals the angle of incidence. But on a curved surface the normal tilts a
little more as you move along the mirror, so rays that arrived parallel leave in
different directions. Concave surfaces tilt the normals towards each other (rays
converge); convex surfaces tilt them apart (rays diverge).

Check it yourself: a steel spoon is both mirrors in one. Its hollow inner side is
concave; its bulging outer side is convex.

Q2 On side-view mirrors of vehicles, there is a warning that says “Objects in mirror are
closer than they appear”. Why is this warning written there?

Because a side-view mirror is a convex mirror, and a convex mirror always makes the image
smaller than the object.
We judge how far away something is largely from how big it looks. A motorcycle whose image is
half the expected size reads to the eye as being twice as far away. So the bus behind you really
is nearer than the small image suggests, and the printed warning is there to stop a driver from
misjudging that gap while changing lanes.

Page 2 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

Why a convex mirror is used at all: because it curves outwards, it gathers light
from a much wider strip of the road behind and squeezes it into the same small
mirror. A plane mirror of the same size would show a correct-sized image but a far
narrower view, leaving a bigger blind spot. The manufacturer accepts the distorted
sense of distance in exchange for the wider field of view — and prints the warning to
cover it.

Did you know? The Hindi edition states the same warning as “वस्तुओ ं के बीच वास्तिवक दूरी
दपण मे ं िदखाई देने वाली दूरी से कम होती है । ” — the actual distance is less than the distance the
mirror suggests. Same idea, said the other way round.

Q3 Why is there a curved line on some reading glasses?

That line is the boundary of a second lens built into the lower part of the same spectacle lens.
Such a lens is called a bifocal — literally two focusing regions in one piece of glass.

The large upper region has a gentle curvature and is used for looking at distant things.
The small lower region, marked off by the curved line, is more strongly curved. Being a
stronger converging (convex) surface, it magnifies near print, so the wearer can read by
simply dropping the eyes.

Why two curvatures are needed: how much a lens bends light depends on how
sharply its surfaces are curved. One curvature cannot suit both a book at 30 cm and
a signboard 30 m away, so the two are ground into one lens and the join shows up
as a visible curved line.

Tip: the chapter itself does not answer this opener — it is meant to make you look
closely at real spectacles. Look at an elder's reading glasses in a slanting light and
the line becomes easy to see.

Q4 Share your questions

Write down the questions the pictures on this page actually raise for you, and keep them at the
front of your notebook — you should be able to answer each one by the end of the chapter.

Page 3 of 63

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Class 8 Science Chapter 10 Light: Mirrors and Lenses
a g l AglaSem · NCERT Solutions

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Sample questions:
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Activity 10.1: Let us explore — Page 153
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10.1 What Are Spherical Mirrors?

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ACTIVITY

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Q1 Take a shiny metallic spoon and hold its curved surface close to your face. Can you

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a Yes, clearly. A polished steel spoon reflects light in the same regular way a mirror does, so an
image of your face is formed in it.
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Why it happens: a polished metal surface is smooth on the scale of light, so a
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parallel beam falling on it stays organised after reflection — every ray obeys the law
of reflection and neighbouring rays stay together. That is what forms an image. A

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dull, scratched spoon scatters the rays in all directions and shows no image, only a

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Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

Q2 Notice the image of your face. Is it different from the image you see in a plane
mirror?

(a) Inner (hollow) side (b) Outer (bulging) side
image is inverted image is erect and smaller

Fig. 10.1, page 153 — your own face seen in a shiny metal spoon. Redrawn sketch of the
book’s photograph: the hollow inner side acts as a concave mirror and turns the face
upside down, while the bulging outer side acts as a convex mirror and gives an erect but
smaller face.

Yes, very different. In a plane mirror your face is the same size and erect. In the hollow inner
side of the spoon, held close, your face looks much larger — and it is still erect.

Why it happens: the inner side of the spoon is a concave surface. Because it curves
inwards, the rays leaving your face are turned inwards on reflection. When your face
is quite close to the spoon those reflected rays still spread apart on the way to your
eye, so your eye traces them back to a point behind the spoon — and the picture
your eye builds there is bigger than your face.

Page 5 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

Q3 While observing the image, slowly move the spoon away from your face. Do you
observe any change in the image?

(a) Inner (hollow) side (b) Outer (bulging) side
image is inverted image is erect and smaller

Fig. 10.1, page 153 — your own face seen in a shiny metal spoon. Redrawn sketch of the
book’s photograph: the hollow inner side acts as a concave mirror and turns the face
upside down, while the bulging outer side acts as a convex mirror and gives an erect but
smaller face.

Yes, and the change happens in a definite order:

1. At first the image stays erect and grows larger.
2. At one particular distance it blurs out and cannot be seen properly.
3. Beyond that distance the image comes back inverted (upside down) — first large, then
getting steadily smaller as the spoon moves farther away.

Why it happens: a concave surface converges the reflected rays. While your face is
nearer than a certain distance the converging is not enough to bring the rays to a
meeting point before they reach your eye, so the image is erect and enlarged. Past
that distance the rays actually cross over in front of the mirror, and an image formed
after the rays have crossed is upside down. The flip is not gradual — it is the crossing
point moving past your eye.

Page 6 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

Q4 Now flip the spoon and repeat the same steps.

With the bulging outer side facing you the behaviour is far simpler. The image is always erect
and always smaller than your face, at every distance. As you move the spoon away the image
shrinks a little more, but it never flips over.

Why it happens: the outer side is a convex surface. It spreads the reflected rays
apart instead of pulling them together, so the rays never cross. Rays that never cross
can never form an inverted image, which is why a convex surface can only ever give
an erect, diminished one.

Try This: a bulging spoon also shows you a strip of the whole room, not just your
face. That extra width is exactly why convex mirrors are used as side-view mirrors.

Activity 10.2: Let us distinguish — Page 155

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Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

10.1 What Are Spherical Mirrors?

ACTIVITY

Q1 Now view them from the side, keeping your eye at their level, to identify whether
the reflecting surface is curved inwards or outwards (Fig. 10.4).

bulges outwards
curves inwards
Eye at mirror level

Concave mirror Convex mirror

Fig. 10.4, page 155 — the two mirrors lying face up on the table, seen from the side with
the eye at their level. Redrawn sketch: the concave mirror’s shiny face is hollowed
inwards, the convex mirror’s bulges outwards.

Looking along the surface, edge-on, the shape shows up at once:

If the shiny surface dips down in the middle — the edges stand higher than the centre — it
is a concave mirror.
If the shiny surface rises in the middle — the centre stands higher than the edges — it is a
convex mirror.

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Class 8 Science Chapter 10 Light: Mirrors and Lenses
a g l AglaSem · NCERT Solutions

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Why the eye must be at surface level: from directly above, both mirrors look like
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flat silver discs, because you are seeing the surface face-on and the small rise or dip
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In-text Questions — Page

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10.1 What Are Spherical Mirrors?

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How can we distinguish between concave and convex mirrors?
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There are three quick tests, and they check three different things.
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Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

TEST CONCAVE MIRROR CONVEX MIRROR

Side view (Activity 10.2) Shiny surface hollowed inwards Shiny surface bulges outwards

Image of an object held close Erect and enlarged Erect and diminished

Object moved far away Image becomes inverted Image stays erect, only shrinks

A parallel beam of light on it Reflected beam converges Reflected beam diverges

Why the image test is the surest one: the shape test can fail if the curvature is very
gentle, but the image test uses the effect of that curvature, which is easy to see. And
the beam test goes to the root of it: converging or diverging is the one property that
follows directly from whether the surface curves in or out.

Tip: a plane mirror gives the third answer — the image stays the same size at every
distance, and a parallel beam stays parallel.

Activity 10.3: Let us explore — Page 155

Page 10 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

10.2 What Are the Characteristics of Images Formed by Spherical Mirrors?

ACTIVITY

Q1 Place the two mirrors side by side in an upright position on a table. Keep the object
in front of them at a small distance (3–4 cm away) as shown in Fig. 10.5a. What kind
of images do you see in each mirror? Are the images of the same size as the object?
Are they erect? Do you see lateral inversion in the images? Write down your
observations in your notebook.

(a) (b)

Concave mirror Convex mirror Concave mirror Convex mirror

Object at a small distance (3–4 cm) Object at a large distance

Fig. 10.5, page 155 — the same toy in front of a concave and a convex mirror. Redrawn
sketch of the book’s photographs. (a) At a small distance the concave mirror gives a
large erect image and the convex mirror a small erect one. (b) Moved far away, the
image in the concave mirror has turned inverted, while the one in the convex mirror is
erect and smaller still.

With the toy only 3–4 cm away:

MIRROR SIZE ERECT OR INVERTED? LATERAL INVERSION?

Concave Enlarged — bigger than the toy Erect Yes

Convex Diminished — smaller than the toy Erect Yes

So the answer to the middle question is no — neither image is the same size as the object, and
that is already a clear difference from a plane mirror. Both images are erect at this small
distance, and both show lateral inversion.

Page 11 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

Why lateral inversion appears in both: lateral inversion is not caused by curvature
at all. It comes from the fact that we are looking at the object from the mirror's side
rather than our own — the side of the toy nearest your right hand is reflected to the
side of the image nearest your left. That reversal happens in a plane, a concave and
a convex mirror alike.

Q2 Now slowly move the object away from the mirrors (Fig. 10.5b). What changes do
you see in the images in both the mirrors? Do the images become smaller or larger?
Do they continue to be erect? Again, note down your observations.

(a) (b)

Concave mirror Convex mirror Concave mirror Convex mirror

Object at a small distance (3–4 cm) Object at a large distance

Fig. 10.5, page 155 — the same toy in front of a concave and a convex mirror. Redrawn
sketch of the book’s photographs. (a) At a small distance the concave mirror gives a
large erect image and the convex mirror a small erect one. (b) Moved far away, the
image in the concave mirror has turned inverted, while the one in the convex mirror is
erect and smaller still.

Concave mirror. The erect image first grows larger. At one particular distance it blurs; past that
distance the image reappears inverted, at first enlarged and then getting steadily smaller the
farther the toy goes.
Convex mirror. Nothing dramatic happens. The image stays erect throughout and only
becomes a little smaller as the toy is moved back.

Page 12 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

Why only the concave mirror flips: a concave mirror converges the reflected rays,
so beyond a certain object distance they actually cross in front of the mirror. Once
light has crossed over, top and bottom are interchanged and the image is inverted. A
convex mirror diverges the reflected rays; they never cross, so its image can never
become inverted, no matter where the object is placed.

Q3 Analyse your observations and draw conclusions.

Conclusion 1. Spherical mirrors do not behave like plane mirrors. A plane mirror always gives
an erect image of exactly the same size; in a spherical mirror the size of the image changes as
the object distance changes.
Conclusion 2. A concave mirror can give an image that is enlarged, of the same size, or
diminished, and erect or inverted — it all depends on how far the object is from the mirror.
Conclusion 3. A convex mirror is far more predictable: its image is always erect and always
diminished, whatever the object distance.
Conclusion 4. Lateral inversion is seen in all three types of mirror, so it cannot be used to tell
them apart.

Tip: when you write conclusions in your notebook, state the condition along with the
result — 'enlarged and erect when the object is close' is a conclusion; 'enlarged' on
its own is not.

In-text Questions — Page 156
10.2 Characteristics of Images Formed by Spherical Mirrors

Q1 Yes. But where do we find concave and convex mirrors being used in our
surroundings?

Each use is chosen for exactly what that mirror does to a beam of light.

Page 13 of 63

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Class 8 Science Chapter 10 Light: Mirrors and Lenses
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MIRROR WHERE IT IS USED WHY THAT MIRROR

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Concave
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Reflector behind a torch bulb;
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Light from the bulb, placed near the focus, leaves

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Concave Dentist's mirror Held close to a tooth it gives an erect, enlarged view

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Convex Side-view mirror of vehicles Erect image and a much wider view of the road

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Convex Road-safety mirror at intersections Lets drivers on both sides see the traffic coming

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Convex Surveillance mirror in big stores

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The pattern behind the list: wherever light has to be gathered and concentrated, or

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a small nearby thing has to be magnified, a concave mirror is used. Wherever a wide

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view matters more than true size, a convex mirror is used.

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agl Q2 Have you ever noticed a dental mirror used by a dentist for inspecting teeth?
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Yes — it is the small round mirror on a long handle that the dentist slips inside the mouth. It is a
concave mirror.
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cavity or crack shows up big enough to examine. The long handle also lets the dentist look at
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the back of a tooth, which cannot be seen directly at all.
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useless for the dentist. That is why the mirror is kept almost touching the tooth.
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A step further — Page 156 a

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Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

10.2 Characteristics of Images Formed by Spherical Mirrors

A STEP FURTHER

Q1 Do you remember learning about the use of telescope in the chapter ‘Beyond Earth’
in Curiosity, Grade 6?

Yes. In 'Beyond Earth' a telescope was introduced as the instrument that lets us see faint and
distant objects in the sky — planets, moons and stars — far better than the unaided eye can.
What this chapter adds is how most modern telescopes do it. They are reflecting telescopes:
the light-collecting element is a large concave mirror, not a lens.

Why a concave mirror is the right choice: a distant star is so far away that its light
arrives as a parallel beam, and a parallel beam is exactly what a concave mirror
concentrates to a point. The bigger the mirror, the more of that faint starlight it
gathers, so the image is brighter and finer detail can be seen. A mirror can also be
supported from behind over its whole area, so it can be built very large without
sagging — which is much harder to do with a lens.

In-text Questions — Page 157
10.3 What Are the Laws of Reflection?

Q1 Do you remember doing the activity for observing the reflection of a beam of light
from a plane mirror?

Yes. In Grade 7 a comb was used as a slit-maker: all its openings were covered with black paper
except one in the middle, and a torch shone through that single slit to give one thin beam of
light travelling along a sheet of white paper. A plane mirror was stood upright on the paper in
the path of that beam.
The beam struck the mirror and came away along a new, definite direction — it did not scatter.
Tilting the mirror or the torch changed the direction of the beam coming away, but there was
always exactly one reflected beam.

Tip: the same set-up is used again in Activity 10.4, this time with the angles actually
measured, and in Activity 10.6 with several slits open so that many parallel beams
fall on the mirror at once.

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Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

Q2 We have observed images formed by three types of mirrors — plane, concave, and
convex. But are there any laws which govern the image formation?

Yes. Everything a mirror does follows from just two laws of reflection, and they apply to every
ray, at every point, on every kind of mirror.

1. The angle of incidence is equal to the angle of reflection, both measured from the normal
at the point of incidence.
2. The incident ray, the normal at the point of incidence, and the reflected ray all lie in the
same plane.

Why two laws are enough: the first law fixes how much the ray is turned; the
second fixes the plane in which it is turned. Together they leave only one possible
direction for the reflected ray. Once the direction of every reflected ray is decided,
the position, size and orientation of the image are decided too — which is why the
same two laws explain the same-size image of a plane mirror and the enlarged,
inverted image of a concave mirror.

Activity 10.4: Let us experiment — Page 157

Page 16 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

10.3 What Are the Laws of Reflection?

ACTIVITY

Q1 Now, move the slit and torch slightly so that the beam of light falls at a different
angle on the mirror (Fig. 10.8b). Does the reflected beam of light also shift?

Plane mirror Plane mirror

Normal Normal

Incident ray Reflected ray Incident ray Reflected ray

Torch Torch
Comb with one slit Comb with one slit

(a) (b)

Fig. 10.8, page 157 — the bench seen from above. A thin beam from the torch passes
through the single slit of the comb and falls on the plane mirror at O. (a) The beam
arrives at one angle; (b) the slit and torch have been moved, so it arrives at a different
angle — and the reflected beam swings round with it, keeping r equal to i.

Yes. The moment the incoming beam falls at a new angle, the reflected beam swings to a new
direction as well — and it does so immediately, not gradually.

Why it happens: the mirror has not moved, so the normal at the point of incidence
still points the same way. Only the incident ray has been turned. Since the reflected
ray must always make the same angle with that normal as the incident ray does,
turning the incident beam through some angle forces the reflected beam to turn
through the same angle — but to the other side of the normal.

Check it yourself: bring the torch closer to the normal and the reflected beam also
comes closer to the normal. Take the torch nearly parallel to the mirror and the
reflected beam skims away nearly parallel to it too.

Page 17 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

Q2 Make the beam of light fall on the mirror at different angles and observe how the
direction of the reflected beam changes.

Whatever angle you choose, the reflected beam always comes off on the other side of the
normal, tilted away from the normal by exactly as much as the incident beam is tilted towards
it.

Normal

Incident ray Reflected ray

i = 40° r = 40°

50°

O
Mirror

Both i and r are measured from the normal, never from the mirror. If i = 40°, then r = 40°, and the
angle between the reflected ray and the mirror surface is 90° − 40° = 50°.

Incident beam close to the normal → reflected beam close to the normal

Incident beam far from the normal → reflected beam far from the normal
In every case, angle of reflection = angle of incidence

Why the normal, and not the mirror, is the reference: the normal is the one line
that is fixed by the mirror surface itself at that point. Measuring from the mirror
would work for a plane mirror but would be useless on a curved one, where 'the
mirror' slopes differently at every point. The normal, drawn at 90° to the surface at
the point of incidence, is defined the same way everywhere — which is why the law
is stated in terms of it.

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Class 8 Science Chapter 10 Light: Mirrors and Lenses
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On your drawing, measure the angle of incidence and the angle of reflection and
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note it in Table 10.1. Repeat the activity several times by changing the angle of
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reflection r are both taken between
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and the mirror.

Table 10.1: Measuring angles of incidence and reflection
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S.NO. ANGLE OF INCIDENCE (I)
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ANGLE OF REFLECTION
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A typical set of readings from a carefully drawn sheet looks like this. Your own numbers will be

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Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

S.NO. ANGLE OF INCIDENCE (I) ANGLE OF REFLECTION (R)

1 20° 20°

2 30° 31°

3 45° 44°

4 60° 60°

5 70° 69°

In every row the two angles are equal to within a degree, and the small differences change sign
from row to row. That is the signature of measurement error, not of a real difference — so the
conclusion is

i = r, the first law of reflection

Where the one-degree gaps come from: the beam from the slit has a real width, so
the pencil line you draw along it may be a little off; the mirror line may not be drawn
exactly where the mirror stood; and a protractor can only be read to about half a
degree. None of these can be removed completely, which is why the readings are
'nearly equal' rather than exactly equal.

Tip: mark two points far apart on the beam before you remove the mirror, then join
them with a ruler. A long line gives a far more accurate angle than a short one.

Q4 Finally, let the incident beam fall on the mirror along the normal and observe the
direction of the reflected beam. What would be the angle of incidence and angle of
reflection in this case?

Both angles are zero.

Incident ray lies along the normal → angle between them = i = 0°

By the law of reflection, r = i = 0°

So the reflected ray also lies along the normal

Page 20 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

The beam therefore travels straight back along the path it came by, retracing itself.

Why this case matters: it is the one direction in which a mirror sends light exactly
back to its source. It is also a useful check on the definition of the angles — if you
had wrongly measured them from the mirror surface you would have got 90° here
instead of 0°, and the law i = r would still have 'worked', which is why the book insists
on measuring from the normal.

In-text Questions — Page 158
10.3 What Are the Laws of Reflection?

Q1 Do you remember learning earlier that the light travels along a straight line?

Yes. In a single uniform medium light travels in straight lines, and that is exactly why we are
allowed to draw it as straight lines with arrows — as rays.
The evidence is all around us:

A shadow has the sharp outline of the object, and a bigger object makes a bigger shadow —
which is what straight-line travel predicts.
Light from a torch passing through a slit shows up as a straight streak on the paper, never a
curved one.
You cannot see round a corner, and a beam through a bent pipe does not come out at the far
end.

Why the ray model is so useful here: once we accept that light travels in straight
lines, a whole diagram can be drawn with a ruler. The path of the light before the
mirror is one straight line, the path after the mirror is another, and the only thing left
to work out is the angle between them — which is what the laws of reflection give
us.

Activity 10.5: Let us experiment — Page 159

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Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

10.3 What Are the Laws of Reflection?

ACTIVITY

Q1 Now, bend the extended part of the sheet along the edge of the table (Fig. 10.10b).
Do you still see the reflected beam on the extended portion?

(a) (b)

Edge of the table Edge of the table

Mirror Mirror

Chart paper Chart paper

Bent
part
reflected beam seen here no beam on the bent part

Fig. 10.10, page 159 — the chart-paper sheet on the table, part of it sticking out beyond
the edge. (a) While the sheet is flat the reflected beam runs right across the extended
part. (b) Fold that part down and the beam is no longer on it — the reflected ray has
stayed in the plane of the incident ray and the normal.

No. The moment the overhanging part of the chart paper is folded down along the edge of the
table, the bright streak on it disappears.

Why it disappears: the reflected beam has not moved and has not switched off — it
is still travelling in exactly the same direction, in the flat plane of the table. The
folded-down paper has swung out of that plane, so the beam no longer lands on it.
Nothing about the light changed; only the screen did.

Check it yourself: hold a sheet of paper flat, level with the table, just beyond the
folded part. The reflected beam shows up on it again, proving the beam was there all
along.

Page 22 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

Q2 Flatten the paper again and observe.

The reflected beam reappears on the extended portion, exactly where it was before.
This appearing-and-disappearing is the whole point of the activity. It shows that the reflected
beam lies in one particular plane — the plane that also contains the incident beam and the
normal — and nowhere else.

Second law of reflection:

The incident ray, the normal to the mirror at the point of incidence,

and the reflected ray all lie in the same plane.

Why the fold is a fair test: bending the paper creates a new plane at an angle to
the old one. If the reflected light were free to leave the original plane, some of it
would fall on the bent portion. Nothing lands there, so the light is confined to the
original plane — which is precisely what the second law states.

In-text Questions — Page 160
10.3 What Are the Laws of Reflection?

Q1 Are laws of reflection applicable to spherical mirrors also?

Yes — the laws of reflection are valid for all kinds of mirrors: plane, concave and convex. They
are laws about a single ray meeting a surface, and a spherical mirror is made of exactly such
surface points.

Why the results still look so different: what changes on a curved mirror is not the
law but the normal. On a plane mirror the normal points the same way at every
point, so a parallel beam stays parallel after reflection. On a spherical mirror the
normal at each point lies along the line joining that point to the centre of the sphere,
so it tilts a little more as you move outwards from the centre of the mirror. Each ray
still turns through i = r about its own normal, but because those normals fan out, the
reflected rays end up converging (concave) or diverging (convex).

Page 23 of 63

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Class 8 Science Chapter 10 Light: Mirrors and Lenses
a g l AglaSem · NCERT Solutions

co m
m.
Tip: this is the key idea of the whole chapter — same law, different normals. Keep it in

m as e
l
mind and every result about spherical mirrors follows.

m .co a g
l a se
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Activity 10.6: Let us explore — Page 160
co m
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10.3 What Are the Laws of Reflection?

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ACTIVITY
g l as
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Q1 Let the multiple parallel beams of light fall upon the plane mirror, concave mirror,
co m
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and convex mirror, one by one. Observe the reflected beams. Is your observation

m as e
.co l
similar to what is shown in Fig. 10.11 (b), (c), and (d)?
a g
se m
g l a
a (b) (c) (d)

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Plane mirror Concave mirror Convex mirror

. c o
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Fig. 10.11 (b), (c), (d), page 160 — many parallel beams from the comb

c o m g l a off a concave mirror
. they close in (converge); off a convex mirror theyaspread out (diverge).
mirror in turn. Off a plane mirror the reflected beams stay parallel;

se m
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a
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.

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Yes, the observations match the figure exactly:

agl
Plane mirror (Fig. 10.11b) — the reflected beams are still parallel to one another.
Concave mirror (Fig. 10.11c) — the reflected beams come closer together, that is, they
converge, and meet at a point.
. c om
Convex mirror (Fig. 10.11d) — the reflected beams spread apart,em
m a s that is, they diverge.

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a

com
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.co


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Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

beams converge beams diverge

F F

Concave mirror Convex mirror

Every single ray still obeys i = r. Because the surface is curved, the normal points in a different
direction at each point of the mirror — and that is what makes the reflected beam close in (concave)
or spread out (convex). The point F, where a parallel beam is brought together — or from which it
seems to spread — is called the focus of the mirror.

Why the three results differ: every one of these beams obeys i = r. On the plane
mirror all the normals are parallel, so every beam is turned through the same angle
and the set stays parallel. On the concave mirror the normals lean inwards as you go
away from the centre, so the outer beams are turned inwards more than the inner
ones and the whole set closes in. On the convex mirror the normals lean outwards,
so the outer beams are thrown further out and the set opens up.

Tip: keep the comb, torch and mirror all standing on the same sheet of paper. If the
torch is lifted even slightly the beams leave the plane of the paper and the pattern
becomes hard to see.

In-text Questions — Page 161
10.3 What Are the Laws of Reflection?

Q1 Since the concave mirror converges the light beam, wouldn’t light get concentrated
in a small area?

Yes, and that is exactly what happens. A concave mirror gathers light falling on its whole surface
and delivers it to a very small patch — its focus.

Page 25 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

Light falling on the mirror: spread over its full area

Light after reflection: crowded into a spot a few millimetres across

→ light energy per unit area at the spot becomes very much greater

Why concentration means heating: light carries energy. A large mirror intercepts a
lot of it. If all of that energy is then dumped on a tiny area of paper, the paper
receives far more energy each second than it can lose to the surrounding air, so its
temperature climbs — until it is hot enough to char and catch fire. Activity 10.7 is
exactly this experiment.

Safety first: never look at the Sun, or at a mirror reflecting it, and never point the
concentrated spot at anyone's face.

Activity 10.7: Let us explore — Page 161

Page 26 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

10.3 What Are the Laws of Reflection?

ACTIVITY SAFETY FIRST

Q1 Adjust the distance of the paper until you get a sharp bright spot on it as shown in
Fig. 10.12.

Sunlight

Bright spot

Concave mirror Sheet of paper

Fig. 10.12, page 161 — redrawn sketch. Sunlight reflected from the concave mirror is
brought together at one small bright spot on the sheet of paper; the distance is
adjusted until that spot is sharpest.

Move the paper slowly towards and away from the mirror. The patch of light first shrinks,
becomes a small, very bright, sharply-edged spot, and then starts to widen again. Stop at the
position where the spot is smallest and brightest.

Why there is one best distance: the Sun is so far away that its rays reach the mirror
effectively parallel. A concave mirror brings a parallel beam together at one
particular point in front of it, its focus. Hold the paper nearer than that and the rays
have not yet met; hold it farther and they have already met and are spreading again.
Only at the focus is all the reflected light squeezed into a single small spot.

Tip: the distance you have just found is the focal length of your mirror. Measure it
from the mirror to the paper and write it in your notebook — deeper mirrors have
shorter focal lengths.

Page 27 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

Q2 Hold the mirror and the sheet of paper steady for a few minutes. Does the paper
start to burn producing smoke?

Yes. Held steady, the paper at the bright spot first darkens to brown, then gives off a thin curl of
smoke, and can finally catch fire.

Why the paper burns: the mirror collects the sunlight falling on its whole face and
pours it onto a spot a few millimetres wide. The energy arriving on that spot each
second is now many times what the same paper would receive lying in the open,
while the paper can still only lose heat to the air as slowly as before. So heat piles up,
the temperature of that tiny patch rises past the temperature at which paper ignites,
and it burns. A plane mirror of the same size cannot do this — it spreads the same
energy over just as large an area as before, so nothing gets hotter.

Safety first: do this only under a teacher's or an adult's supervision, never look
towards the Sun or into the mirror, focus the light only on paper, and keep some
water at hand.

A step further — Page 161
10.3 What Are the Laws of Reflection?

A STEP FURTHER

Q1 Do you remember learning in an earlier chapter, about electric furnaces for melting
steel?

Yes — in Chapter 4, Electricity: Magnetic and Heating Effects. There we learnt that the heating
effect of electric current is used in a specially designed high-temperature furnace, an enclosed
space built to generate heat, in which scrap steel is melted and recycled into usable steel.
A solar furnace reaches the same very high temperature by a completely different route.
Instead of pushing a current through a conductor, a large concave mirror (or a field of mirrors)
concentrates sunlight onto a small target.

Page 28 of 63

as e
Class 8 Science Chapter 10 Light: Mirrors and Lenses
a g l AglaSem · NCERT Solutions

co m
m.
ELECTRIC FURNACE SOLAR FURNACE

as e
comes from .c
om
Where the energy Electric current Sunlight
a g l
a s em
gl heat is produced
aHow Heating effect of current in a Reflection — sunlight concentrated by mirrors
conductor onto a small area

co m No fuel; works only while the Sun shines
Running cost and fuel
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Needs a continuous electricity ag
supply
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Why the comparison is worth making: both furnaces do the same job — crowd a
co m
lot of energy into a small space. The electric furnace does it by choosing where the
e m.
m l as
.co g
current meets resistance; the solar furnace does it by choosing where the light rays

em a
s
meet. The mirror in a solar concentrator is doing on a huge scale exactly what your

gl a
a
small concave mirror did to a sheet of paper in Activity 10.7.

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In-text Questions — Page 162sem
a
10.4 What Is a Lens? agl

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We explored the images of an object formed by curved mirrors. But how do objects
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Q1

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look when viewed through transparent materials with curved surfaces?
a g
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agl ANSWER

m
They look changed in size, and if the surface is curved enough they can even look upside down

a se
com l
— much as they do in a curved mirror, except that now we are looking through the material
. a g
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ase
rather than in it.

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Why there is any change at all: light bends when it passes from air into glass or

m
water and again when it comes back out. At a flat surface every ray of a beam is bent

. co
m
in the same way, so the beam carries on with its shape unaltered and the object

o m l a se
looks normal. At a curved surface the two faces meet the beam at a different slant at
ag ones — the beam is
.cevery point, so the outer rays are bent more than the inner
m squeezed together or opened out, and the object we see is magnified or shrunk.
l a se
ag
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Check it yourself: look at this line of print through a glass tumbler full of water.
a g la
Through the flat rim nothingmchanges; through the curved side the letters stretch.
l a se
ag

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Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

Q2 But would those objects continue to look the same if the surface of the transparent
material is curved?

No, they would not.
Through a flat window pane every object keeps its size and shape — that is why we can use
windows at all. The moment the transparent surface is curved, the letters or objects behind it
look larger or smaller than they really are. Activity 10.8 shows this with nothing more than a
drop of water.

Why flat glass is the special case: at a flat sheet, the ray bends on the way in and
bends back by exactly the same amount on the way out, because the two faces are
parallel. The ray therefore leaves travelling in its original direction — shifted a little
sideways, but not turned. Curved faces are not parallel, so the two bends do not
cancel, and the direction of the ray really is changed.

Activity 10.8: Let us explore — Page 162
10.4 What Is a Lens?

ACTIVITY

Q1 Examine the water drop. What is the shape of its surface? Is it flat or curved inward
or curved outward?

The surface of the drop is curved outward — it bulges upward like a tiny dome. It is neither flat
nor hollowed inwards.

Why the thin film of oil or wax matters: on bare glass water spreads out into a flat
film, and a flat film would change nothing. Oil or wax stops the water from wetting
the glass, so the drop pulls itself into a rounded bead instead of spreading — and it
is that outward bulge which makes it behave like a lens.

Tip: a smaller drop bulges more sharply than a large one, and a more sharply curved
surface magnifies more. Try two drops of different sizes side by side.

Page 30 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

Q2 Now, look down through the water drop at the text below. Do you find some change
in the size of the letters just below the water drop? Do they look enlarged or
smaller?

Yes — the letters under the drop look clearly enlarged, bigger than the very same letters lying
just outside the drop.

Why the drop magnifies: the drop is a piece of transparent material with a surface
curved outward — thicker at its middle than at its edge. That is precisely the shape
of a convex lens. It bends the light coming up from the printed letter inwards, and
your eye, tracing those bent rays back in straight lines, places the letter at a size
larger than it really is.

Did you know? A magnifying glass (Fig. 10.14) is doing exactly what this drop of
water does — it is simply a bigger, permanent version made of glass.

In-text Questions — Page 163
10.4 What Is a Lens?

Q1 What changes can be seen in the objects when viewed through lenses?

The object can change in size and, with a convex lens, even in orientation.

LENS OBJECT CLOSE TO THE LENS OBJECT TAKEN FAR FROM THE LENS

Convex Erect and enlarged Inverted — first enlarged, then getting smaller

Concave Erect and diminished Still erect, becoming smaller

Why a convex lens can flip the object but a concave lens cannot: a convex lens
converges the light passing through it, so beyond a certain object distance the rays
actually cross before reaching your eye — and crossed rays give an inverted view. A
concave lens only ever spreads the rays out; they never cross, so what you see can
never be upside down.

Page 31 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

Q2 Do lenses also converge or diverge the light beam?

Yes. A convex lens converges a parallel beam of light and a concave lens diverges it — which
is why they are also called a converging lens and a diverging lens. Activity 10.10 shows both.

Why the two shapes behave oppositely: a ray is bent more where the two faces of
the glass are more steeply slanted to each other. In a convex lens the glass is
thickest in the middle, and its faces slope inwards as you go outwards from the
centre, so the outer rays are pushed inwards and the beam closes on a point. In a
concave lens the glass is thinnest in the middle and the faces slope the other way, so
the outer rays are pushed outwards and the beam opens up.

Activity 10.9: Let us experiment — Page 163

Page 32 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

10.4 What Is a Lens?

ACTIVITY

Q1 Look at the object through the lens from the other side of the lens (Fig. 10.17a) and
note your observations in your notebook.

Convex lens

(a) object close to the lens

Fig. 10.17a, page 163 — redrawn sketch. Looking through a convex lens at an object
placed close behind it: the object looks erect and enlarged.

With the object a small distance behind the convex lens, it appears erect and enlarged —
clearly bigger than it looks with the naked eye, and the right way up.

Why it is enlarged at this distance: the convex lens bends the rays leaving the
object inwards, but over such a short distance they are still spreading apart when
they reach your eye. The eye traces them back along straight lines to a point farther
away and wider apart than the object itself — so the object looks bigger, and the
right way up.

Page 33 of 63

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Class 8 Science Chapter 10 Light: Mirrors and Lenses
a g l AglaSem · NCERT Solutions

co m
m.
Tip: record the actual distance in centimetres alongside every observation. Without

m as e
l
the distance, the observation cannot be compared with anyone else's.

m .co a g
l a se
a g
Q2 Now slowly move the object farther from the lens and keep observing how the

co m
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image changes (Fig. 10.17b). How does the distance of the object from the convex

m .
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lens affect how it looks?

g l as
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co m
em.
m l as
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l a se
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m a s
m .co agl
l a se Convex lens
a g

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se m
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a (b) object far from the lens
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com g l a
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s looks inverted (its wooden block appears at the top).
Fig. 10.17b, page 163 — redrawn sketch. The same convex lens with the object moved far
behind it: now the a
a g l object

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m l
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The distance decides everything:
a g
a s em1. Small distance — erect and enlarged.
agl 2. At one particular distance — the view blurs and no clear image can be seen.
.c
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3. Beyond that distance — the object appears inverted, at first still enlarged.

m . co
4. Farther still — it stays inverted and now steadily diminishes in size.
e agl
g l as
a

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m .
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Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

Why the flip happens: the convex lens always converges the light. When the object
is close, the rays are spreading so strongly that the converging is not enough to
bring them together before your eye — the view stays erect. Past a certain object
distance the rays do meet and cross over on their way to you, and once light has
crossed, up and down are interchanged. That is the point at which the picture turns
over.

Q3 Now repeat the steps using a concave lens (Fig. 10.17c).

Concave lens

(c) concave lens

Fig. 10.17c, page 163 — redrawn sketch. Through a concave lens the object always looks
erect and diminished.

With a concave lens the answer is the same at every distance: the object appears erect and
diminished. As you move it farther from the lens it just keeps getting smaller. It never flips over
and it is never enlarged.

Page 35 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

Why nothing else can happen: a concave lens is thinner in the middle, so it always
makes the rays passing through it spread apart more than they were already
spreading. Rays that are pushed apart can never meet and cross, and without a
crossing there can be no inverted view and no enlargement. This is the lens
counterpart of the convex mirror.

Q4 Analyse your observations recorded in your notebook and compare the images seen
through both lenses. What conclusions do you draw?

CONVEX LENS CONCAVE LENS

Shape Thicker at the middle Thicker at the edges

Object at a small distance Erect, enlarged Erect, diminished

Object at a large distance Inverted, then diminishing Erect, diminishing

Effect on a parallel beam Converges it Diverges it

Behaves like which mirror? Concave mirror Convex mirror

Conclusion. The image formed by a convex lens can be enlarged, diminished or of the same
size, and erect or inverted, depending on the distance of the object from the lens. The image
formed by a concave lens is always erect and diminished.

The single idea behind the whole table: converging optics (concave mirror, convex
lens) can make rays cross, so they can produce inverted and enlarged images.
Diverging optics (convex mirror, concave lens) never let rays cross, so they are locked
into erect, diminished images.

In-text Questions — Page 164

Page 36 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

10.4 What Is a Lens?

Q1 Since convex lens converges a light beam, can it also burn a paper?

Yes. Anything that concentrates sunlight into a small enough spot can char and ignite paper,
and a convex lens converges light just as a concave mirror does.
The only difference is the route the light takes. The concave mirror sends the sunlight back to a
focus in front of itself; the convex lens lets the sunlight pass through and brings it to a focus on
the far side. In both cases the energy falling on the whole face of the mirror or lens ends up on
a spot a few millimetres wide. Activity 10.11 is exactly this test.

Safety first: never look at the Sun directly or through the lens — it can damage your
eyes permanently.

Activity 10.10: Let us investigate — Page 164
10.4 What Is a Lens?

ACTIVITY

Q1 Now let the multiple parallel beams of light fall upon the thin glass plate, convex
lens, and concave lens one by one as shown in Fig. 10.18. Does the parallel beam of
light pass through as it is in all three cases?

(a) (b) (c)

Thin glass plate Convex lens Concave lens

Fig. 10.18, page 164 — the same set of parallel beams sent through each in turn.
Through the thin glass plate they come out parallel, exactly as they went in; the convex
lens brings them together; the concave lens spreads them apart.

No — only the flat plate leaves the beam alone.

Page 37 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

Thin transparent glass plate — the beams pass through as they are and are still parallel
on the far side.
Convex lens — the beams are brought together; they converge and meet at a point.
Concave lens — the beams are spread apart; they diverge.

F F

Convex lens — converging Concave lens — diverging

Light bends where it enters the glass and again where it leaves. A convex lens is thicker in the middle,
so it bends the outer rays inwards and they meet at F. A concave lens is thinner in the middle, so it
bends the rays outwards and they only appear to come from F. The point F is called the focus of the
lens.

Why the flat plate changes nothing: its two faces are parallel, so a ray is bent one
way going in and bent back by exactly the same amount coming out. It emerges
travelling in its original direction — displaced very slightly sideways, but not turned.
In a lens the faces are curved and therefore not parallel, so the two bends do not
cancel, and how much they fail to cancel depends on how far the ray is from the
centre. That is what turns a parallel beam into a converging or a diverging one.

Q2 Record and analyse your observations.

WHAT THE LIGHT PASSED THROUGH WHAT THE EMERGING BEAMS DID NAME

Thin transparent glass plate Stayed parallel — passed through as it is —

Convex lens Came together at a point Converging lens

Concave lens Spread out Diverging lens

Analysis. A convex lens is also called a converging lens and a concave lens a diverging lens.
Comparing with Activity 10.6, the convex lens does to transmitted light what the concave mirror
does to reflected light, and the concave lens does what the convex mirror does.

Page 38 of 63

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Class 8 Science Chapter 10 Light: Mirrors and Lenses
a g l AglaSem · NCERT Solutions

co m
m.
Tip: keep the paper sheets on both books at the same level and the room fairly dark.

m as e
l
The converging point of the convex lens is easy to miss if the beams are faint.

m .co a g
l a se
a g
Activity 10.11: Let us investigate — Page 165
co m
. ag
10.4 What Is a Lens?

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ACTIVITY SAFETY FIRST
g l as
a
Q1 Repeat Activity 10.7 by putting a convex lens in the path of sunrays in place of a
co m
m.
concave mirror (Fig. 10.19). Could you burn the paper?

m as e
.co a g l
se m
g l a
a
m a s
m .co agl
Sunlight
l a se
a g
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Convex lens
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m l as
m .co a g
l a se
ag
se m
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Bright spot

. a g l
m
ase
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Sheet of paper

Fig. 10.19, page 165 — redrawn sketch. A convex lens held in the path of the sunrays

co m
.
converges them to one bright spot on the paper, exactly as the concave mirror did in

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as
Activity 10.7.
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.c

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Yes. Hold the convex lens so that sunlight passes through it onto the paper, and adjust the

. co a gl
distance until the patch of light shrinks to the smallest, brightest spot. Held steady there for a

s m
e smokes and can catch fire — exactly as it did with the concave
a
agl
minute or two, the paper browns,
mirror.

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Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

Why the lens does the job just as well: the Sun's rays arrive parallel. A convex lens
brings a parallel beam together at its focus, so all the light energy that fell on the
whole face of the lens is delivered to a spot a few millimetres across. The energy
arriving there each second far exceeds what the paper can carry away to the air, so
its temperature rises until it ignites. The mirror does this by reflection and the lens
by refraction, but the physical reason — concentration of light energy — is identical.

Safety first: do not look at the Sun directly or through the lens; it may damage your
eyes. Work under adult supervision and keep water nearby.

In-text Questions — Page 165
10.4 What Is a Lens?

Q1 Where all are the lenses used?

Lenses are used almost everywhere we need to control where light goes.

WHERE WHAT THE LENS DOES THERE

Eyeglasses (spectacles) Bend the light a little before it enters the eye, so that a clear image is formed
for the wearer

Cameras, including smartphone Converge light from the scene to form a sharp image on the sensor
cameras

Telescopes Gather light from distant objects in the sky

Microscopes Give a greatly enlarged view of very small objects

Magnifying glass Makes small print appear bigger

Inside our own eye A convex lens that can change its shape, letting us look at a book and then at
something far away

Page 40 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

Why the eye lens is the most remarkable of them all: every lens in the list above
has one fixed curvature, so it converges light by one fixed amount. The lens in our
eye can change its shape — becoming more curved for near objects and flatter for
distant ones — so a single lens does the work of a whole set. That is how we read a
line of print and then look up at a hill without changing anything.

Keep the curiosity alive — Page 166
End-of-chapter questions

KEEP THE CURIOSITY ALIVE

Q1 A light ray is incident on a mirror and gets reflected by it (Fig. 10.21). The angle
made by the incident ray with the normal to the mirror is 40°. What is the angle
made by the reflected ray with the mirror? (i) 40° (ii) 50° (iii) 45° (iv) 60°

Incident ray Normal Reflected ray

i r

Mirror

Fig. 10.21, page 166 — a light ray striking a mirror and bouncing off it. The dashed line is
the normal; i and r are both measured from it.

(ii) 50°

Page 41 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

Angle of incidence, i = 40° (measured from the normal)

By the first law of reflection, r = i = 40°

The normal is at 90° to the mirror, so
angle with the mirror = 90° − r = 90° − 40° = 50°

Normal

Incident ray Reflected ray

i = 40° r = 40°

50°

O
Mirror

Both i and r are measured from the normal, never from the mirror. If i = 40°, then r = 40°, and the
angle between the reflected ray and the mirror surface is 90° − 40° = 50°.

Why 40° is the trap: the question gives the angle from the normal but asks for the
angle from the mirror. The law i = r is about the normal only. Once r = 40° is found,
the remaining step is simple geometry — the normal and the mirror are
perpendicular, so the two angles at the point of incidence on that side must add up
to 90°.

Tip: before answering any reflection question, underline whether the angle
mentioned is measured from the normal or from the mirror. Almost every mistake in
this topic starts there.

Page 42 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

Q2 Fig. 10.22 shows three different situations where a light ray falls on a mirror: (i) The
light ray falls along the normal. (ii) The mirror is tilted, but the light ray still falls
along the normal to the tilted surface. (iii) The mirror is tilted, and the light ray falls
at an angle of 20° from the normal. Draw the reflected ray in each case (Use a ruler
and protractor for accurate drawing). What is the angle of reflection in each case?

Incident ray
Incident ray Incident ray
Normal

20°

Mirror

Mirror Mirror

(i) (ii) (iii)

Fig. 10.22, page 166 — the three situations. (i) A horizontal mirror with the ray falling
straight down along the normal. (ii) The mirror tilted through 20°, with the ray tilted
with it so that it is still along the normal. (iii) The same tilted mirror, but the ray falls
vertically, making 20° with the normal (dashed).

Draw the normal first in every case — at 90° to the mirror surface at the point where the ray
strikes it. Then measure i from that normal and set r equal to it on the other side.

Incident Reflected Incident Incident
Normal
Reflected

Mirror
Mirror Mirror
(i) i = 0°, r = 0° (ii) i = 0°, r = 0° (iii) i = 20°, r = 20°

The reflected ray in each case. In (i) and (ii) the ray arrives along the normal, so i = 0° and the ray
simply returns along its own path. In (iii) the reflected ray lies on the other side of the normal, also at
20°.

Page 43 of 63

as e
Class 8 Science Chapter 10 Light: Mirrors and Lenses
a g l AglaSem · NCERT Solutions

co m
m.
CASE ANGLE OF ANGLE OF PATH OF THE REFLECTED RAY

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INCIDENCE (I) REFLECTION (R)

. a g l
(i)
s e m0° 0° Straight back along the incident ray
a
a(ii)gl 0° 0° Straight back along the incident ray

m
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(iii) 20° 20° At 20° on the other side of the normal

sem
(40° away from the incident ray)

a
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Why (i) and (ii) give the same answer even though the mirrors are differently

c o m
placed: tilting the mirror tilts its normal by the same amount. In (ii) the ray has been
.
tilted along with the mirror, so the ray is still along the normal and i ismstill 0°. What
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answer is not how the mirror is placed in the room but l
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between

a g l

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Check it yourself: in (iii) the incident and reflected rays are 40° apart, that is 2 × 20°.
a s
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This is always true — the angle between the incident and the reflected ray is twice
agl
the angle of incidence.
l a se
a g

co m
m .
as e
Q3 In Fig. 10.23, the cap of a sketch pen is placed in front of three types of mirrors.
m l
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Match each image with the correct mirror.
m a
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ag Image Object

se m
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m . a
ase
agl

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(i) (ii)
a g l (iii)

se m
g l a Fig. 10.23, page 166 — redrawn sketch of the three photographs. In each one the actual
a c
.
sketch-pen cap stands beside the mirror at the same size, so you can compare it directly
with its image inside the mirror.
s e m
m a
e m . co agl
g l as
ANSWER a
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Compare each image in the mirror with the actual cap standing beside it.

. co
e m
m l as
.co a g
Page 44 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

IMAGE WHAT IT LOOKS LIKE MIRROR

(i) Erect, but much smaller than the cap Convex mirror

(ii) Erect, and much larger than the cap Concave mirror

(iii) Erect, and the same size as the cap Plane mirror

Why size alone settles it here: the cap is close to each mirror, and at close range
the three mirrors are easy to separate. A plane mirror can only ever give a same-size
image. A convex mirror can only ever give a diminished one. A concave mirror, with
the object this close, gives an enlarged one. Since all three images are erect,
orientation gives no clue — the size does.

Tip: if the cap in (ii) had been moved much farther back, the concave mirror would
have shown it upside down. The concave mirror is the only one of the three that can
do that.

Page 45 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

Q4 In Fig. 10.24, the cap of a sketch pen is placed behind a convex lens, a concave lens,
and a flat transparent glass piece — all at the same distance. Match each image
with the correct type of lens or glass.

(i) (ii) (iii)

Fig. 10.24, page 167 — redrawn sketch. The same sketch-pen cap stands the same
distance behind each of the three — two round lenses and one flat square glass piece —
and is viewed through them.

All three pictures are taken at the same object distance, so any difference in size is caused by
the glass alone.

IMAGE WHAT IT LOOKS LIKE LENS/GLASS TYPE

(i) Erect and much enlarged, seen through a round lens Convex lens

(ii) Erect and much smaller, seen through a round lens Concave lens

(iii) Unchanged in size, seen through a flat square plate Flat transparent glass piece

Why the flat plate leaves the cap unchanged: its two faces are parallel, so every
ray is bent one way entering the glass and bent back by the same amount leaving it.
The ray carries on in its original direction, only shifted slightly sideways, so the cap is
seen at its true size. The two lenses have curved, non-parallel faces, so the bends do
not cancel — the convex lens converges the light and enlarges the cap at this
distance, and the concave lens diverges it and shrinks the cap.

Page 46 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

Q5 When the light is incident along the normal on the mirror, which of the following
statements is true: (i) Angle of incidence is 90° (ii) Angle of incidence is 0° (iii) Angle
of reflection is 90° (iv) No reflection of light takes place in this case

(ii) Angle of incidence is 0°

The angle of incidence is the angle between the incident ray and the normal.

Here the incident ray lies along the normal, so the angle between them is i = 0°

and therefore r = i = 0°

The light is reflected straight back along its own path.

Why the other options fail: 90° would be the angle the ray makes with the mirror
surface, not with the normal — options (i) and (iii) come from measuring against the
wrong line. Option (iv) is simply wrong: the mirror reflects the light perfectly well; it
just sends it back the way it came, so there is no separate reflected beam to see.

Page 47 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

Q6 Three mirrors — plane, concave and convex are placed in Fig. 10.25. On the basis of
the images of the graph sheet formed in the mirrors, identify the mirrors and write
their names above the mirrors.

Graph sheet

? ? ?

Fig. 10.25, page 167 — redrawn sketch. A graph sheet stands behind three round mirrors
lying on the table; each mirror shows its own image of the same grid. Compare the size
of the squares in each mirror with the squares on the sheet itself, then write the name
of the mirror in the box above it.

Compare the squares seen in each mirror with the squares of the real graph sheet standing
behind them.

MIRROR IN FIG. WHAT THE REFLECTED GRAPH SHEET LOOKS LIKE MIRROR
10.25

Left Squares are bigger than on the real sheet, so fewer of them Concave
fit in the mirror mirror

Middle Squares are the same size as on the real sheet and the lines Plane mirror
stay straight

Right Squares are smaller, many more of them fit in, and the lines Convex
are visibly bowed mirror

Page 48 of 63

as e
Class 8 Science Chapter 10 Light: Mirrors and Lenses
a g l AglaSem · NCERT Solutions

co m
m.
Why a graph sheet is such a good test object: a plain object only tells you its

m l a se
overall size. A grid tells you what the mirror does at every point at once — if the
o g the mirror must
.c the edge are a different size from those at the centre,
a
m
squares near
be a
l se and the bending of the straight grid lines shows which way. In the middle
g the grid is reproduced exactly, which no curved mirror can do; on the right
curved,
amirror
the grid is both shrunk and bowed, the signature of a surface curving outwards.
co m
e m . ag
g l as
a
Check it yourself: the convex mirror also shows a wider strip of the graph sheet
than the other two. Squeezing a wider view into the same disc is exactly why the
squares in it look small.
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l a se
a g
m a s
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co m
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m ase
.co


a g l Page 49 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

Q7 In a museum, a woman walks towards a large concave mirror (Fig. 10.26). She will
see that: (i) her erect image keeps decreasing in size. (ii) her inverted image keeps
decreasing in size. (iii) her inverted image keeps increasing in size and eventually it
becomes erect and magnified. (iv) her erect image keeps increasing in size.

large concave mirror she walks towards the mirror

Fig. 10.26, page 167 — redrawn sketch. In the museum a woman stands well beyond the
focus of a large concave mirror, and at that distance her image in it is inverted. Think
about what happens to that image as she keeps walking towards the mirror.

(iii) her inverted image keeps increasing in size and eventually it becomes erect and
magnified.
Follow the walk from a long way off:

1. Far from the mirror — the image is inverted and small, exactly as in Fig. 10.26 where she
is shown upside down in the mirror.
2. Walking closer — the image stays inverted but grows larger.
3. Very close to the mirror — the image turns over and becomes erect and magnified.

Page 50 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

I

C F O

mirror

Concave mirror with the object between F and the mirror. The two reflected rays spread apart, so
they never meet in front of the mirror; extended backwards (dashed) they meet behind it. The image
is therefore virtual — it cannot be caught on a screen — and it is erect and enlarged.

Why the flip happens on the way in: a concave mirror converges the reflected rays.
While she is far away, those rays cross in front of the mirror before reaching her eye,
and light that has crossed shows her upside down. As she comes nearer, the rays
leaving her are spreading more steeply, and past a certain distance the mirror can
no longer bring them to a crossing at all — they still diverge on the way to her eye,
which traces them back to a point behind the mirror. That backward-traced image is
erect and larger than her, and because no light actually arrives from behind the
mirror it cannot be caught on a screen; such an image is called a virtual image.

Tip: options (i) and (iv) can be ruled out immediately — an image that stays erect the
whole time is what a plane or a convex mirror gives, not a concave one.

Q8 Hold a magnifying glass over text and identify the distance where you can see the
text bigger than they are written. Now move it away from the text. What do you
notice? Which type of lens is a magnifying glass?

A magnifying glass is a convex lens — a lens that is thicker at the middle than at its edges.
What you notice, step by step:

1. Held close to the page, the letters look erect and enlarged; there is a range of a few
centimetres over which the print is both big and sharp.

Page 51 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

2. As the lens is lifted, the letters get bigger still, up to a point.
3. At one particular height the print blurs completely and cannot be read.
4. Lift it further and the print reappears upside down and now gets smaller the higher you go.

Why the letters flip: a convex lens converges the light from the page. While the
page is nearer than a certain distance the rays are still diverging when they reach
your eye, so the eye traces them back to an enlarged, erect image. Beyond that
distance the rays converge and cross before reaching your eye, and crossed rays
give an inverted view. The blurred position in between is where the rays are neither
properly diverging nor properly crossed.

Check it yourself: hold the magnifying glass at arm's length and look at a distant
window through it. You will see the window upside down and tiny — which is the last
step of the list above.

Q9 Match the entries in Column I with those in Column II. Column I: (i) Concave mirror
(ii) Convex mirror (iii) Convex lens (iv) Concave lens. Column II: (a) Spherical mirror
with a reflecting surface that curves inwards. (b) It forms an image which is always
erect and diminished in size. (c) Object placed behind it may appear inverted at
some distance. (d) Object placed behind it always appears diminished in size.

COLUMN I COLUMN WHY
II

(i) Concave (a) By definition — its reflecting surface curves inwards
mirror

(ii) Convex (b) Its reflected rays always diverge, so the image can only be erect and
mirror diminished

(iii) Convex lens (c) It converges light, so beyond a certain distance the rays cross and the
object appears inverted

(iv) Concave (d) It diverges light, so the object seen through it is always erect and
lens diminished

Page 52 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

How to be sure of (b) and (d): both entries say 'always', and only diverging optics
can promise 'always'. Entry (b) says forms an image — the language of a mirror — so
it must be the convex mirror. Entry (d) says object placed behind it — the language
used for lenses in this chapter, since we look through a lens — so it must be the
concave lens.

Q10 The following question is based on Assertion/Reason. Assertion: Convex mirrors
are preferred for observing the traffic behind us. Reason: Convex mirrors provide a
significantly larger view area than plane mirrors. Choose the correct option: (i)
Both Assertion and Reason are correct and Reason is the correct explanation for
Assertion. (ii) Both Assertion and Reason are correct but Reason is not the correct
explanation for Assertion. (iii) Assertion is correct but Reason is incorrect. (iv) Both
Assertion and Reason are incorrect.

(i) Both Assertion and Reason are correct and Reason is the correct explanation for
Assertion.
Assertion — correct. Side-view mirrors on vehicles are convex mirrors.
Reason — correct. Because a convex mirror curves outwards, it collects light from a much
wider strip of the road behind and shows it in the same small mirror.
And the Reason really is the explanation. A driver needs to see as much as possible of the
road behind before changing lanes, and the widest view in the smallest mirror is precisely what
a convex mirror gives. That is why it is chosen over a plane mirror of the same size.

The price paid for the wide view: squeezing a wider scene into the same mirror
makes every vehicle look smaller, and a smaller image reads as a more distant one.
This is exactly the reason the warning 'Objects in mirror are closer than they appear'
is printed on the mirror. It is a drawback of the same property, not a different
property — so it does not weaken the Reason.

Page 53 of 63

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Class 8 Science Chapter 10 Light: Mirrors and Lenses
a g l AglaSem · NCERT Solutions

co m
m.
In Fig. 10.27, note that O stands for object, M for mirror, and I for image. Which of
se
Q11

o m l a
the following statements is true? (i) Figure (a) indicates a plane mirror and Figure
gmirror and Figure
m .c
(b) indicates a
a concave mirror. (ii) Figure (a) indicates a convex

a e indicates a concave mirror. (iii) Figure (a) indicates a concave mirror and Figure
s(b)
agl (b) indicates a convex mirror. (iv) Figure (a) indicates a plane mirror and Figure (b)
indicates a convex mirror.

co m
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g l as
a

co m
em.
m l as
m .co g
a I
ase
O M

agl (a)

m a s
m .co agl
l a se
a g
O M I
co m
(b)
m .
o m l a se
m .c Fig. 10.27, page 168 — O is the object, M the mirror andagI the image. Compare the
a s e
agl
height of I with the height of O in each figure.

se m
com g l a
m . a
ase

a gl
(ii) Figure (a) indicates a convex mirror and Figure (b) indicates a concave mirror.
Compare the arrow marked I with the arrow marked O in each figure. In both figures the image
is erect and lies on the far side of the mirror M, so only the size can tell them apart.
co m
m .
m as e
.co
FIGURE IMAGE COMPARED WITH
g l
WHICH MIRROR CAN DO THIS
a
a s em OBJECT

agl
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(a) Erect and smaller than O Convex mirror — its image is always erect and
diminished
s e m
m a
e m . co Concave mirror, with the object close to it agl
as
(b) Erect and larger than O

a g l

co m
m .
m as e
.co


a g l Page 54 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

I

O F

mirror

Convex mirror. The reflected rays always diverge, so the image is found by extending them
backwards. It is virtual — it cannot be caught on a screen — and it is erect and diminished; it always
lies between the mirror and F — closer to the mirror than the object, however far away the object is
taken.

Why a plane mirror is ruled out in both: a plane mirror gives an image exactly
equal in size to the object, and in neither figure is I the same height as O. So options
(i) and (iv), which both call one of the figures a plane mirror, cannot be right. Option
(iii) has the two the wrong way round — enlargement is the concave mirror's doing,
not the convex mirror's.

Tip: in both figures I is drawn on the opposite side of M from O — that is, behind the
mirror. Both are therefore erect images traced back behind the mirror, which is the
only kind a convex mirror ever makes and the kind a concave mirror makes when
the object is close.

Page 55 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

Q12 Place a pencil behind a transparent glass tumbler (Fig. 10.28a). Now fill the
tumbler halfway with water (Fig. 10.28b). How does the pencil appear when viewed
through the water? Explain why its shape appears changed.

Pencil

Water

(a) (b)

Fig. 10.28, page 169 — redrawn sketch. (a) The pencil behind the empty tumbler. (b) The
tumbler half filled with water — through the water the pencil looks distinctly broader,
so it seems to break at the water surface.

Through the empty tumbler the pencil looks normal. Once the tumbler is half filled, the part of
the pencil seen through the water looks distinctly broader than the part above the water level,
and it is shifted a little to one side — so the pencil seems to break at the water surface.

Page 56 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

wider

(a) empty (b) half filled with water

Through the empty tumbler the pencil looks normal. Through the water the lower part looks broader
and shifted sideways, so the pencil seems to break at the water surface.

Why the shape appears changed: the tumbler filled with water is a transparent
body with a curved outer surface — a cylindrical convex lens lying on its side. Light
coming from the pencil bends as it passes from water and glass into air. Because the
surface is curved, rays leaving from the left and the right edges of the pencil are
bent inwards by different amounts, so they reach your eye more widely separated
than they started. Your eye traces them back in straight lines and places the pencil
wider than it really is. Above the water line there is only air and a thin curved sheet
of glass, which bends the light hardly at all — so that part is seen at its true width,
and the two parts no longer line up.

Try This: a curved surface only magnifies across the curve. Turn the tumbler in your
hand and the pencil widens sideways but never gets taller — because the tumbler is
curved only around its sides, not up and down.

Discover, design, and debate — Page 169

Page 57 of 63

Class 8 Science Chapter 10 Light: Mirrors and Lenses AglaSem · NCERT Solutions

Extension tasks

DISCOVER, DESIGN, AND DEBATE

Q1 Visit a nearby hospital or the clinic of an ENT specialist, or a dentist, with your
teacher or parents. Request the doctor to show you the mirrors used for examining
ear, nose, throat, and teeth. Identify the kind of mirror used in these instruments.

How to do it. Fix the visit through your teacher. Carry a notebook and, for each instrument you
are shown, record three things: the shape of the reflecting surface seen edge-on, whether the
image of a small object held close to it looks bigger or smaller, and what the doctor uses that
instrument for.
What you will find.

INSTRUMENT KIND OF WHY THAT KIND
MIRROR

Dental mirror (mouth Concave Held close to a tooth it gives an erect, enlarged view, so a
mirror) small cavity can be seen clearly. Its long handle also lets
the dentist see the back of a tooth.

ENT head mirror worn on Concave, with a It converges light from a lamp behind the patient onto
the forehead small hole at its the ear, nose or throat, lighting up a deep, narrow space.
centre The doctor looks through the central hole along the same
line as the light.

Small laryngeal / Plane or slightly It is used mainly to turn the line of sight round a corner,
examination mirror on a concave into the throat; a plane surface keeps the view
stem undistorted.

The common idea: in medical instruments a concave mirror is chosen when light
has to be gathered into a dark cavity or a small structure has to be magnified. Where
the mirror is only needed to look round a corner, a plane surface is better, because it
does not change the size of what the doctor sees.

Tip: ask the doctor why the head mirror has a hole in the middle. It is the neatest
example in the whole chapter of light being sent one way and viewed along the
same line.

Page 58 of 63

as e
Class 8 Science Chapter 10 Light: Mirrors and Lenses
a g l AglaSem · NCERT Solutions

co m
m.
Harnessing sunlight is key to solving future energy challenges. In devices like solar
se
Q2

India,.c o m l a
cookers (Fig. 10.29), mirrors are used to converge sunlight and generate heat. In
gand reducing fossil
m a
such designs are used in villages, thus saving electricity

l a se use. Think of a design for a solar cooker for your school or home and prepare a
fuel
a g detailed proposal for it including the budget required.

co m
e m . ag
as
Sunlight

Cooking pot a g l

co m
em.
m l as
m .co a g
l a se
a g
m a s
m .co agl
l a se
a g

co mStand
m .
as e
com
Curved mirror strips

. a g l
m
ase
Fig. 10.29, page 169 — redrawn sketch of the solar cooker in the photograph. Curved

agl
mirror strips set in a tilted frame collect the sunlight and send it up to a single small
area, where the cooking pot is held on an arm.

se m
com g l a
m . a
gl ase
a

What a good proposal must contain: (1) the need it meets, (2) the physics it uses, (3) a labelled
design with dimensions, (4) a materials-and-cost table, (5) how you will test it, and (6) its
co m
limitations.
m .
m as e
.co g l
Sample proposal — a box-type solar cooker with a plane reflector for the school kitchen
a
s e m1. Need. The school heats water and cooks pulses for the midday meal on an LPG stove. A solar
agla cooker used on the roughly 250 clear days in a year can take over part of that work and cut both
.c
m
the gas bill and the fuel used.

m a s e
co agl
2. Physics used. A hinged plane mirror lid reflects an extra beam of sunlight into the box, so the

m .
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food gets sunlight from two directions instead of one. A blackened inner surface absorbs that

g l as
light and turns it into heat, and a double glass cover lets sunlight in but traps the warm air, so
a
the temperature inside climbs to about 100–120 °C.

co m
m .
m ase
.co


a g l Page 59 of 63