Question

what is the wave theory in optics ?

Answer 1

The wave theory of light says that light is essentially a transverse electromagnetic wave and propagates in space as wavefronts. Wave front construction is done by Huygen's construction under which a point in the primary wavefront acts as a source of the secondary wavefronts. And secondary wavefronts give rise to the next primary wavefront, that is how the wave propagates. It explains refraction, reflection, interference, diffraction, polarization and other electro-optic effects like Kerr Effect etc

Answer 2

Wave

A wave is a disturbance which propagates energy (and momentum) from one place to the other without the transport of matter. It is well spread over a region of space without clear cut boundaries.

Phase

Phase is defined as position of a point on the wave, phase is usually denoted in angular measurements such as , /2, 3/2, 2.



Crest and Trough

Crest : It is the highest point between two successive waves.

Trough : It is the lowest point between two successive waves.



Wavefront and Rays

A wavefront is defined as a surface joining the points of same phase. The speed with which the wavefront moves outwards from the source is called the phase velocity or wave velocity.



The energy of the wave moves in the direction perpendicular to the wavefront. The energy travels outwards along straight lines emerging from the source. These lines are called the rays.

The shape of a wavefront is usually determined by the geometry of the source so we have different wavefronts .

Spherical Wavefront

When the source of light is a point source, the wavefront is a sphere with center at the source. Spherical wavefront progresses to larger sphere and larger sphere and this continues.



Cylindrical Wavefront

When the source of light is linear e.g. a slit as shown in figure, all the points equidistant from the source lie on a cylinder. Therefore, the wave front is cylindrical. Cylindrical wavefront progresses to large cylinder and larger cylinder and this continues.



Plane Wavefront

When the point source or linear source of light is at very large distance, a small portion of spherical or cylindrical wavefront appears to be plane. Such wavefront is called a plane wavefront.



Concave Wavefront

Concave wavefront is created using a convex lens, when a plane wavefront passes through a convex lens it changes its shape, the wavefront is formed normal to the converging rays hence a concave wavefront is formed.



Convex Wavefront

Convex wavefront is created using a concave lens, when a plane wavefront passes through a concave lens it changes its shape, the wavefront is formed normal to the diverging rays hence a convex wavefront is formed.