Wave optics, also termed as Physic optics is the branch of optics which
deals with various phenomenon including diffraction, polarization of light,
interference pattern in optics and solved problems, Young’s double slit
experiment etc. It is an eminent branch which assumes great importance as a
large number of questions in various competitions are generally asked from it.
We shall discuss some of the chief topics of wave optics for IIT JEE here
in brief as they have been discussed in detail in the coming sections:
Huygens’ Wave Theory of Light:
This is one of the most important principles of wave analysis which was
introduced by an eminent physicist Huygens. The crux of the principle is that
every point of a wave front can be treated as the source of secondary wavelets
that spread out in all directions with the same speed as that of the speed of
propagation of waves. This just implies that an edge of a wave can actually be
viewed as creating a series of circular waves. Usually, these waves combine to
carry on the propagation, but sometimes there are noteworthy evident results.
The wave front which is defined as the surface, on which the wave disturbance is
in phase, actually appears to be tangent to all the circular waves.
Though these results can be easily derived from Maxwell’s equations as
well, but Huygens’ Principle is better suited for performing calculations on
waves. Maxwell was the one who provided solid theoretical basis to what Huygens’
had already anticipated around two centuries back.
According to this principle, a plane light wave passes through free space
at the speed of light. The below figure demonstrates the motion of light rays
associated with the propagation of wave front. They move in straight lines as
A critical description of the Huygens’ Principle is:
(i) Each point on a wave front acts as a source of a new disturbance and
therefore emits its own set of spherical waves which are called secondary
wavelets. The secondary wavelets travel in all directions with the velocity of
light as long as they move in the same medium.
(ii) The locus or the envelope of these wavelets in the forward direction
indicates the position of new wave front at any subsequent time.
Diffraction basically refers to the bending of light around hindrances.
This basically means that it creates some sort of interference in the passage of
light. Another associated concept is of a diffraction grating. A diffraction
grating refers to the screen with a bunch of parallel slits which are placed at
a distance‘d’ from each other. Diffraction is in fact a special case of
interference. It takes place when a wave hits against the barrier of an edge.
The passing of light through some edge or gap is involved in almost all optical
phenomena which clearly imply that diffraction takes place in almost all of
them, though the impact might be negligible. A wave tends to bend around the
hindrance as a result of diffraction. Diffraction can also be used for studying
the structure of particular objects. It is even possible to reverse and move
backward from the diffraction pattern to know about the nature of the object.
Young’s Double –Slit Experiment
As a result of double slit experiment by Thomas Young in 1801, the wave
theory of light came into the limelight. The double-slit experiment is based on
the doctrines of constructive interference and destructive interference and
hence proves that light resembles some of the properties of waves.
The experiment involves throwing up of light on a screen containing two
narrow slits separated by a distance ‘d’. At a distance ‘L’ from the first
screen, a second screen is placed and the light which passes through the two
slits shines on falling on this screen.
It is apparent form the figure that the light of single wavelength λ falls on
the first screen and since the slits are narrower than λ, so the light passes
and spreads all over the second screen. As depicted in the figure above the
point P on the back screen is the point which receives light from two different