NCERT Book Class 11 Physics Chapter 14 Waves

CHAPTER FOURTEEN

WAVES

14.1 INTRODUCTION
In the previous Chapter, we studied the motion of objects
oscillating in isolation. What happens in a system, which is
a collection of such objects? A material medium provides
14.1 Introduction
such an example. Here, elastic forces bind the constituents
14.2 Transverse and to each other and, therefore, the motion of one affects that of
longitudinal waves
the other. If you drop a little pebble in a pond of still water,
14.3 Displacement relation in a
the water surface gets disturbed. The disturbance does not
progressive wave
remain confined to one place, but propagates outward along
14.4 The speed of a travelling
a circle. If you continue dropping pebbles in the pond, you
wave
see circles rapidly moving outward from the point where the
14.5 The principle of
water surface is disturbed. It gives a feeling as if the water is
superposition of waves
moving outward from the point of disturbance. If you put
14.6 Reflection of waves
some cork pieces on the disturbed surface, it is seen that
14.7 Beats
the cork pieces move up and down but do not move away
Summary
from the centre of disturbance. This shows that the water
Points to ponder
mass does not flow outward with the circles, but rather a
Exercises
moving disturbance is created. Similarly, when we speak,
the sound moves outward from us, without any flow of air
from one part of the medium to another. The disturbances
produced in air are much less obvious and only our ears or
a microphone can detect them. These patterns, which move
without the actual physical transfer or flow of matter as a
whole, are called waves. In this Chapter, we will study such
waves.
Waves transport energy and the pattern of disturbance has
information that propagate from one point to another. All our
communications essentially depend on transmission of sig-
nals through waves. Speech means production of sound
waves in air and hearing amounts to their detection. Often,
communication involves different kinds of waves. For exam-
ple, sound waves may be first converted into an electric cur-
rent signal which in turn may generate an electromagnetic
wave that may be transmitted by an optical cable or via a