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 shown here:
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:
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 sources.
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