
PHYSICS368
satellite. Detection of the original signal will usu-
ally involve these steps in reverse order.
Not all waves require a medium for their
propagation. We know that light waves can
travel through vacuum. The light emitted by
stars, which are hundreds of light years away,
reaches us through inter-stellar space, which
is practically a vacuum.
The most familiar type of waves such as waves
on a string, water waves, sound waves, seismic
waves, etc. is the so-called mechanical waves.
These waves require a medium for propagation,
they cannot propagate through vacuum. They
involve oscillations of constituent particles and
depend on the elastic properties of the medium.
The electromagnetic waves that you will learn
in Class XII are a different type of wave.
Electromagnetic waves do not necessarily require
a medium - they can travel through vacuum.
Light, radiowaves, X-rays, are all electromagnetic
waves. In vacuum, all electromagnetic waves
have the same speed c, whose value is :
c = 299, 792, 458 ms
–1
. (15.1)
A third kind of wave is the so-called Matter
waves. They are associated with constituents of
matter : electrons, protons, neutrons, atoms and
molecules. They arise in quantum mechanical
description of nature that you will learn in your
later studies. Though conceptually more abstract
than mechanical or electro-magnetic waves, they
have already found applications in several
devices basic to modern technology; matter
waves associated with electrons are employed
in electron microscopes.
In this chapter we will study mechanical
waves, which require a material medium for
their propagation.
The aesthetic influence of waves on art and
literature is seen from very early times; yet the
first scientific analysis of wave motion dates back
to the seventeenth century. Some of the famous
scientists associated with the physics of wave
motion are Christiaan Huygens (1629-1695),
Robert Hooke and Isaac Newton. The
understanding of physics of waves followed the
physics of oscillations of masses tied to springs
and physics of the simple pendulum. Waves in
elastic media are intimately connected with
harmonic oscillations. (Stretched strings, coiled
springs, air, etc., are examples of elastic media).
We shall illustrate this connection through
simple examples.
Consider a collection of springs connected to
one another as shown in Fig. 15.1. If the spring
at one end is pulled suddenly and released, the
disturbance travels to the other end. What has
happened? The first spring is disturbed from its
equilibrium length. Since the second spring is
connected to the first, it is also stretched or
compressed, and so on. The disturbance moves
from one end to the other; but each spring only
executes small oscillations about its equilibrium
position. As a practical example of this situation,
consider a stationary train at a railway station.
Different bogies of the train are coupled to each
other through a spring coupling. When an
engine is attached at one end, it gives a push to
the bogie next to it; this push is transmitted from
one bogie to another without the entire train
being bodily displaced.
Now let us consider the propagation of sound
waves in air. As the wave passes through air, it
compresses or expands a small region of air. This
causes a change in the density of that region,
say
δρ
, this change induces a change in pressure,
δ
p, in that region. Pressure is force per unit area,
so there is a restoring force proportional to
the disturbance, just like in a spring. In this
case, the quantity similar to extension or
compression of the spring is the change in
density. If a region is compressed, the molecules
in that region are packed together, and they tend
to move out to the adjoining region, thereby
increasing the density or creating compression
in the adjoining region. Consequently, the air
in the first region undergoes rarefaction. If a
region is comparatively rarefied the surrounding
air will rush in making the rarefaction move to
the adjoining region. Thus, the compression or
rarefaction moves from one region to another,
making the propagation of a disturbance
possible in air.
Fig. 15.1 A collection of springs connected to each
other. The end A is pulled suddenly
generating a disturbance, which then
propagates to the other end.