CBSE Class 12 Physics Question Paper 2020 Set 55-3-1 Solutions

Strictly Confidential: (For Internal and Restricted use only)
Senior School Certificate Examination-2020
Marking Scheme – PHYSICS THEORY (042)
(55/3/1)
General Instructions: -

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Page 1 of 18

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mark is correctly and clearly indicated. It should merely be a line. Same is with
the X for incorrect answer.)
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awarded.

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should be marked as cross (X) and awarded zero (0)Marks.

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error detected by the candidate shall damage the prestige of all the personnel
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on payment of the processing charges.

Page 2 of 18

MARKING SCHEME: PHYSICS
QUESTION PAPER CODE: 55/3/1
Q.No. Value Points/Expected Answer Marks Total
Marks
SECTION A
1 (D) 1 1

energy will be provided by external source displacing the charge.
2 (A) 1 1

1
∈𝑜

3 (A) 1 1

𝐶1
𝐶2

4 (C) 1 1

Decreases with increase in its conductivity
5 (B) 1 1

Mobility
6 (D) 1 1

𝑃
4

7 (D) 1 1

1
𝑛2

8 (C) 1 1

heavily doped n-side as well as p-side
9 (D) 1 1

Helix
10 (D) 1 1

-F
11 Cylindrical 1 1
12 Divergent lens/ Concave lens 1 1
13 Two 1 1
14 √3 1 1
15 Intensity 1 1
OR
ℎ(𝜐 − 𝜐𝑜 )
16 Z=R 1 1
Alternatively, Impedance=Resistance
17 Copper 1 1
Page 3 of 18

18 Zero 1 1
Eddy currents are produced in metal block / block gets heated
19 J.C Bose observed / produced electromagnetic waves of short 1 1
wavelength/ did very significant work in production of e.m waves.
20 X rays are used as diagnostic tool in medicine / 1 1
Gamma rays are used to destroy cancer cells.
SECTION B
21
Writing formula
𝐸1 ∝ 𝑙1 ½ mark
𝐸1 − 𝐸2 ∝ 𝑙2 ½ mark
𝐸
Calculating 1 1 mark
𝐸2

𝐸1 ∝ 𝑙1 ½

𝐸1 − 𝐸2 ∝ 𝑙2 ½

𝐸1 − 𝐸2 𝑙2
=
𝐸1 𝑙1

𝐸2 𝑙2
1− = ½
𝐸1 𝑙1

𝐸2 𝑙2 1 2
= 1− = 1− =
𝐸1 𝑙1 3 3

𝐸1 3
= ½ 2
𝐸2 2
22
Modification in magnetic field pattern by paramagnetic
material 1 mark
Modification in magnetic field pattern by diamagnetic
material 1 mark

1+1 2

diamagnetic paramagnetic

Page 4 of 18

23
Deducing the expression for Mutual Inductance: 2 marks

The given system has the shape shown here
Let a current I flow through the larger coil.

½

Magnetic field, due to the current at the centre of coil is

𝜇𝑜 𝐼𝑁2 ½
𝐵𝐶 =
2𝑟2
We can consider this to be the value of the magnetic field over the
whole area of the smaller coil(as r1<<r2)
∴Magnetic flux through the smaller coil

𝜇𝑜 𝐼𝑁2 2
= 𝐵𝑐 (𝜋𝑟12 )𝑁1 = 𝜋𝑟1 𝑁1
2𝑟2
𝜇𝑜 𝜋𝑟12
=( 𝑁 𝑁 )𝐼 ½
2𝑟2 1 2
But Magnetic flux=MI
Where M= mutual Inductance of the system
𝜇𝑜 𝜋𝑟12 𝑁1 𝑁2
∴ 𝑀𝐼 = 𝐼
2𝑟2
2
𝜇𝑜 𝜋𝑟12 𝑁1 𝑁2 ½
𝑀=
2𝑟2
24
Radiation of electromagnetic wave by an oscillating charge
1 mark
Relation between the frequency of radiated wave and the
frequency of oscillating charge 1 mark

An oscillating charge produces an oscillating electric field in space,
which produces an oscillating magnetic field, which in turn, is a
source of oscillating electric field, and so on. The oscillating electric
and magnetic fields thus regenerate each other, as the wave 1
propagates through the space.

The frequency of the electromagnetic wave equals the frequency of 1 2
oscillation of the charge.

Page 5 of 18

OR

a) Explaining the fact that e.m waves carry energy
1 mark
b) Correct Explanation 1 mark

a) Consider a plane perpendicular to the direction of propagation of
the electromagnetic wave. If there are, on this plane, electric
charges, they will be set and sustained in motion by the electric and 1
magnetic fields of the electromagnetic wave. The charges thus
acquire energy and momentum from the waves.

b) When the sun shines on your hand, you feel the energy being
absorbed from the electromagnetic waves (your hands get warm). 1 2
Electromagnetic waves also transfer momentum to your hand but
because c is very large, the amount of momentum transferred is
extremely small and you do not feel the pressure.

[For any other alternative correct explanation also, award full 2
marks]
25
Determining power of the combination 1 ½ mark
Nature of combination ½ mark

1 1 1
= −
𝑓 𝑓1 𝑓2
½
1 𝑓2 − 𝑓1
=
𝑓 𝑓1 𝑓2 ½
𝑓2 − 𝑓1 ½
∴𝑃=
𝑓1 𝑓2

Because 𝑓2 < 𝑓1 ∴ P is negative
½ 2
∴ nature is diverging lens

OR

Writing the formula 1 mark
(a) effect of wavelength on Resolving power ½ mark
(b) effect of diameter of lens on Resolving power ½ mark

Resolving power of compound microscope is
1
2𝜇 𝑠𝑖𝑛𝜃
𝑅𝑒𝑠𝑜𝑙𝑣𝑖𝑛𝑔 𝑃𝑜𝑤𝑒𝑟 =
1.22𝜆

Page 6 of 18

Justification of the following is based on the above formula: ½
a) If 𝜆 decreases, Resolving Power increases. ½ 2
b) If diameter of objective lens is increased, 𝑠𝑖𝑛𝜃 increases,
Resolving Power increases
26

a) definition of threshold frequency ½ mark
b) definition of stopping potential ½ mark
incorporating these terms in Einstein’s photoelectric
equation 1 mark

(a) Threshold Frequency: The minimum cut off frequency 𝜐𝑜 ½
below which no photoelectric emission is possible, even if the
intensity is large

(b) Stopping Potential: The minimum negative (retarding) ½
potential 𝑉𝑜 given to the plate for which the photocurrent stops or
becomes zero is called the cut off or stopping potential.

1 2
½
ℎ𝜈 = 𝜙0 + 𝑚𝑉𝑚𝑎𝑥
2

ℎ𝜈 = ℎ𝜈𝑜 + 𝑒𝑉𝑜 ½ 2

27
a) Stating the number of spectral lines ½ mark
Showing the transitions in energy level diagram 1 mark
b) Stating the transition for the shortest wave length emission
½ mark
½
a) number of spectral lines =6
energy level diagram

1

b) n=4 to n=1
½ 2
SECTION C
28
a) differentiating between random velocity and drift
velocity 1mark
Order of magnitude 1 mark
b) drawing the graph showing the variation of drift
velocity as a function of Current density 1 mark

Page 7 of 18

a) Write any one difference

Random Velocity 𝜐 Drift Velocity 𝑣𝑑
1. The velocity acquired by the 1. The average velocity
free electrons in the absence of acquired by the free electrons
electric field. in presence of electric field. 1
2. The average random 2. The average drift velocity is
velocity is zero. not zero.
3. Has quite a large value 3. Has a very small value

Order of magnitude of random velocity is 102m/s. ½
Order of magnitude of drift velocity is 10-3m/s. ½

[Note: If the student writes drift speed is nearly 10-5 times smaller
than random velocity ,award the last 1 mark]

3
1

𝐼 𝑛 𝑒 𝐴 𝜈𝑑
[if a student writes 𝐽= = = 𝑛𝑒𝜈𝑑 but does not draw
𝐴 𝐴
the graph award ½ mark only]
29
a) writing the formula for resonant angular frequency
½ mark
calculating this angular frequency 1 mark
b) writing the formula for Q value ½ mark
calculating Q value 1 mark

a)
1 ½
𝜔𝑜 =
√𝐿𝐶

1 ½
=
√2 × 32 × 10−6

=125 rad/s ½

b)
1 𝐿 𝐿𝜔
𝑄= √ 𝑜𝑟 𝑄=
𝑅 𝐶 𝑅 ½

1 2 2 125
𝑄= √ = 25 Alternatively Q=  25 1 3
10 32×10−6 10

Page 8 of 18

OR

a) Calculating rms value of current 1 mark
calculating peak value of current 1 mark
b) Phase difference between current through inductor and
applied voltage ½ mark
change in phase difference ½ mark

a)
𝑋𝐿 = 𝜔𝐿 = 2𝜋𝜈𝐿
5 ½
∴ 𝑋𝐿 = 2𝜋 × 50 × = 500 Ω
𝜋
200 2
𝐼𝑟𝑚𝑠 = = = 0.4𝐴 ½
500 5

𝐼0 = √2 𝐼𝑟𝑚𝑠
½
= √2 × 0.4
= 0.56 𝐴 ½
[Even if student expresses the answer as (0.4√2)𝐴 give the last ½
marks]
𝜋
b) 𝑜𝑟 900 ½ 3
2
decreases ½
30
a) Ray diagram for concave mirror ½ mark
derivation of mirror formula 2 marks
b) Correct explanation ½ mark

a) Ray diagrams for concave mirror

½

Derivation of Mirror Formula

From the diagram,
∆𝐴′ 𝐵′ 𝐹 & ∆𝑀𝑃𝐹 are similar

𝐵′ 𝐴′ 𝐵′ 𝐹
∴ =
𝑃𝑀 𝐹𝑃

Page 9 of 18

𝐵′ 𝐴′ 𝐵′ 𝐹
= (∵ 𝑃𝑀 = 𝐴𝐵) − − − − − − − 𝑒𝑞1 ½
𝐵𝐴 𝐹𝑃

Since
∠𝐴𝑃𝐵 = ∠𝐴′ 𝑃𝐵′

∆𝐴′ 𝐵′ 𝑃 & ∆𝐴𝐵𝑃 are also similar

𝐵′ 𝐴′ 𝐵′ 𝑃
= − − − − − − − − − 𝑒𝑞 2 ½
𝐵𝐴 𝐵𝑃

Comparing eq. 1 and eq. 2

𝐵′ 𝑃 𝐵′ 𝑃 − 𝐹𝑃
=
𝐵𝑃 𝐹𝑃

As per the sign convention
𝐵′ 𝑃 = −𝜈, 𝐹𝑃 = −𝑓, BP = -u

−𝑣 + 𝑓 −𝑣 𝑣
= =
−𝑓 −𝑢 𝑢
−𝑣𝑢 + 𝑢𝑓 = −𝑣𝑓
Dividing by uvf
1 1 1
⇒ + = 1
𝑣 𝑢 𝑓
b) Magnification is different for different object distances ½ 3
31
a) Explaining the high nuclear density 1 mark
b) Explaining the non-Colombian nature 1 mark
c) Drawing the graph 1 mark
a) Volume of Nucleus is very small but its mass is almost the total
mass of the atom
𝑀𝑎𝑠𝑠
𝑁𝑜𝑤 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 =
𝑉𝑜𝑙𝑢𝑚𝑒

That is why density of nucleus is very high. 1
Alternatively, the matter consisting of atoms, has a very large
amount of empty space.

b) Nuclear forces are very strong, attractive and independent of 1
charge and are short ranged.
Whereas Colombian Force are charge dependent and long range.
(Accept any one point of difference)

1 3

Page 10 of 18

32
Meaning of wave nature of electron 1 mark
Explaining the quantisation of angular momentum using
de Broglie hypothesis 2 marks

Moving electron can show wave characteristics. 1

½

From the diagram
2𝜋𝑟 = 𝑛𝜆 ½
(Note: Award one mark here even if the student just writes this
equation without drawing the diagram)
According to de Broglie
ℎ
𝜆= ½
𝑝
𝑛ℎ
∴ 2𝜋𝑟 = 𝑛𝜆 =
𝑝
𝑛ℎ
2𝜋𝑟 =
𝑚𝜈
𝑛ℎ
𝑚𝜈𝑟 = 𝑤ℎ𝑒𝑟𝑒 𝑛 = 1,2,3, … .. 3
2𝜋
This explains the quantisation of angular momentum of the ½
orbiting electron.
33
Naming the diode ½ mark
Labelled circuit diagram 1 mark
Working 1 mark
V-I characteristics ½ mark

Zener diode ½

1

Page 11 of 18

If the input voltage increases, the current through Rs and Zener
diode also increases. This increases the voltage drop across Rs
without any change in the voltage across the Zener diode. This is
because in the breakdown region, Zener voltage remains constant 1
even though the current through the Zener diode changes.

½
3

34
a) Stating the reason for adding impurity atoms ½ mark
b) Naming the two processes 1 mark
Explaining the two processes 1 mark
Creation of potential barrier ½ mark

a) To increase the electrical conductivity / to increase the number
density of charge carriers ½

b) Diffusion and Drift ½+½

Explanation
Diffusion: During the formation of p-n junction, due to the
concentration gradient across the p and n sides, the motion of ½
majority charge carriers give rise to diffusion current.

Drift: Due to the electric field developed at the junction, the
motion of the minority charge carriers due to electric field is called ½
drift.

With the passage of time, diffusion current decreases whereas drift
current increases and balance each other. This, creates a potential ½
barrier. 3

Page 12 of 18

SECTION D
35
a) Diagram ½ mark
Derivation 1 ½ mark
Orientation for maximum and half of the maximum
torque ½ + ½ mark

b) Formula ½ mark
Calculation 1 mark
Result ½ mark

a)

½

From diagram
½
Magnitude of Torque= (𝑞𝐸)(2𝑎 𝑠𝑖𝑛𝜃)
= (2𝑞𝑎)(𝐸 𝑠𝑖𝑛𝜃)
½
= 𝑝𝐸𝑠𝑖𝑛𝜃
For direction

𝜏⃗ = 𝑝⃗ × 𝐸⃗⃗ ½

i) for maximum Torque, dipole should be placed perpendicular to
the direction of electric field
𝜋
𝜃 = 900 = ½
2

ii) For the torque to be half the maximum,
𝜋
𝜃 = 300 =
6 ½
(b)

𝑘𝑞
𝐸𝑃𝐴 = 𝐸𝑃𝐵 ; 𝐸=
𝑟2
𝑘𝑞𝐴 𝑘𝑞𝐵
=
𝑥2 (2 − 𝑥)2 ½
1 4 ½
2
=
𝑥 2 − 𝑥)2

1 2
= ½
𝑥 2−𝑥

Page 13 of 18

2 ½
𝑥= 𝑚
3 5

OR

a) Derivation of expression for energy stored 2 ½ marks
Form of energy stored ½ mark
b) formula 1 mark
calculation ½ mark
result ½ mark

a) Work done in adding a charge dq = 𝑑𝑊
= 𝑉𝑑𝑞 ½
𝑞
= 𝑑𝑞
𝑐 ½
∴Total Amount of work(W )in charging a capacitor

1 𝑄 ½
𝑊 = ∫ 𝑑𝑊 = ∫ 𝑞𝑑𝑞
𝐶 0

𝑄2
𝑊= ½
2𝐶

(𝐶𝑉)2 1
= = 𝐶𝑉 2 ½
2𝐶 2

The electrostatic Energy/ potential energy is stored in the electric
field between the plates. ½
b) 𝐶 = 1𝜇𝐹 = 1 × 10−6 𝐹; 𝑉 = 10 𝑣𝑜𝑙𝑡
𝑄 = 𝐶𝑉 1
= 1 × 10−6 × 10 ½
= 10−5 𝑐𝑜𝑢𝑙𝑜𝑚𝑏 ½ 5
36
a) Diagram showing direction ½ mark
Derivation of expression 1 ½ marks
Definition of 1 ampere 1 mark
b) Magnetic fields of the wires ½ mark
Net magnetic field and its direction 1 + ½ mark

a)

½

Page 14 of 18

𝜇𝑜 𝐼1 ½
𝐵1 =
2𝜋𝑑

𝐹⃗ = 𝐼(𝑙⃗ × 𝐵
⃗⃗ )

𝐹21 = 𝐼2 𝑙2 𝐵1 𝑠𝑖𝑛90𝑜

𝜇𝑜 𝐼1 ½
= 𝐼2 𝑙2
2𝜋𝑑
Force per unit length
𝐹21 𝜇𝑜 𝐼1 𝐼2 ½
𝑓21 = =
𝑙2 2𝜋𝑑

Definition of 1 ampere – One ampere is defied as that steady current
which, when maintained in each of the two very long, straight
parallel conductors of negligible cross section, and placed at a 1
distance 1 meter apart in vacuum, will produce on each of the
conductors a force equal to 2 × 10−7 𝑁 per metre of length.
𝐹
Alternatively, 𝐼1 = 𝐼2 = 𝐼𝐴, 𝑑 = 1𝑚, = 2 × 10−7 𝑁/𝑚
𝑙

b)

⃗⃗ = ⃗⃗⃗⃗⃗ ½
𝐵 𝐵1 + ⃗⃗⃗⃗⃗
𝐵2

𝜇𝑜 𝐼1 𝜇𝑜 𝐼2
𝐵= +
2𝜋𝑟1 2𝜋𝑟2

𝜇𝑜 3 3
= ( + )
2𝜋 6 × 10−2 6 × 10−2

4𝜋 × 10−7 × 3
=
𝜋 × 6 × 10−2

= 2 × 10−5 𝑡𝑒𝑠𝑙𝑎 1
Direction of 𝐵⃗⃗ at midpoint is perpendicular to the plane containing 5
the two conductors and pointing downwards. ½
(Note: give full credit of this direction if student takes direction
opposite to the shown in fig and answer accordingly)

Page 15 of 18

OR

a) Diagram 1 mark
explaining the shape of the path 2 marks
b) formula ½ mark
calculation 1 mark
result ½ mark

a)

1

Inside the dee, the magnetic field makes the charged particle to ½
move in semi-circular path.
Electric field between the dees accelerates the charged particle. ½
The sign of Electric field is changed in tune with the circular
motion of the particle. ½
Each time, the acceleration increases the energy of the particle.
As the energy increases, radius of circular path increases.
So, the path is spiral. ½

b)
𝑉
𝑅= −𝐺 ½
𝑖𝑔
2𝑉
𝑅1 = − 𝐺 = 𝑅𝑜 − 𝐺
𝑖𝑔
𝑅1 + 𝐺 = 2𝑅𝑜 ½
𝑉
[𝑊ℎ𝑒𝑟𝑒 𝑅𝑜 = ]
𝑖𝑔
Similarily
𝑅2 + 𝐺 = 𝑅𝑜
𝑅3 + 𝐺 = 𝑅𝑜 /2 ½
From the above equations,
𝑅1 − 𝑅2 = 2(𝑅2 − 𝑅3 5
𝑅1 − 3𝑅2 + 2𝑅3 = 0 ½

Page 16 of 18

37
a) Meaning of plane polarised light 1 mark
Diagram ½ mark
Derivation of the relationship between 𝜇 𝑎𝑛𝑑 𝜃 1 ½ marks
b) Each graph 1+1 marks

a) A light whose electric vector direction does not change with
time is a plane polarised light.
Alternatively, if electric vector is confined to one particular plane,
containing direction of propagation it is referred to as plane 1
polarized light.

½

sin 𝑖
𝜇=
sin 𝑟 ½

𝑠𝑖𝑛𝜃
= 𝜋 = 𝑖𝑓 𝑖 = 𝜃
sin ( − 𝜃) ½
2

𝑠𝑖𝑛𝜃 ½
= = tan 𝜃
𝑐𝑜𝑠𝜃

b) (i) (ii)

1+1 5

[Note: also accept if a student plots (ii) graph as follows ]

Page 17 of 18

OR

a) description of experiment with diagram 1 mark
derivation of the expression for fringe width 2 marks
b) finding the wavelength of refracted light 1 mark
finding the speed of refracted light 1 mark

a)

½

S is a monochromatic source of light. S1 and S2 are two pinholes
separated by a distance d. GG' is the screen placed at the distance
½
D from the pinholes.
P is a general point on the screen.
Derivation
2 2 2
𝑑 2 2
𝑑 2 ½
(𝑆2 𝑃) − (𝑆1 𝑃) = [𝐷 + (𝑥 + ) ] − [𝐷 + (𝑥 − ) ]
2 2
𝑑2 𝑑2
= 𝐷2 + 𝑥 2 + + 𝑥𝑑 − 𝐷2 − 𝑥 2 − + 𝑥𝑑
4 4
= 2𝑥𝑑
2𝑥𝑑 2𝑥𝑑
𝑝𝑎𝑡ℎ 𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒 = 𝑆2 𝑃 − 𝑆1 𝑃 = ≈ ½
𝑆2 𝑃 + 𝑆1 𝑃 2𝐷
𝑥𝑑
𝑃𝑎𝑡ℎ 𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒 =
𝐷
For maxima
𝑥𝑑 ½
= 𝑛𝜆, 𝑛 = 0,1,2 …
𝐷
𝑛𝜆𝐷
𝑜𝑟 𝑥𝑛 =
𝑑
(𝑛 + 1)𝜆𝐷
𝑥𝑛+1 =
𝑑
𝛽 = 𝑥𝑛+1 − 𝑥𝑛
𝜆𝐷 ½
𝛽=
𝑑
b) 5
𝑐0 𝜐𝜆0 𝜆0
𝜇𝑤 = = = ½
𝑐𝑤 𝜐𝜆𝑤 𝜆𝑤
𝜆0 588 × 3
𝜆𝑤 = = = 441 𝑛𝑚
𝜇𝑤 4 ½
𝑐0 3 × 108 × 3
𝑐𝑤 = = = 2.25 × 108 𝑚/𝑠 ½+½
𝜇𝑤 4

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