Wave front and wave normal

Wavefront and Wave Normal
Concept of wavefront :
Consider a point source ‘S’ of light placed at a point in air or vacuum. The point source of light will emit the light waves in all directions with same velocity in the same medium. Let ‘v’ be the velocity of light in air medium. In time ‘t’ all the waves covers a distance equal to ‘vt’. In the other words in time ‘t’ all the waves will reach on a spherical surface of radius vt and centre S. The spherical surface so formed is called wavefront.

It should be noted that all points on the spherical surface should be in the same state of vibration.
Definition of wavefront :
“The locus of all the points of the medium to which all the waves will reach simultaneously, so that all the points are in the same state of vibration is called wavefront”

Types of wavefronts:

There are three types of wavefronts.
1. Spherical wavefront.
2. Plane wavefront.
3. Cylindrical wavefront.

1. Spherical wavefront : A wavefront originating from the point source of light at finite distance is called spherical wavefront.


2. Plane wavefront : As time passes the spherical surface of spherical wavefront is so large that a small part of it can be considered as a plane wavefront.
A wavefront originating from the point source of light at infinite distance i.e. at very large distance is called as plane wavefront. The plane wavefront can be obtained by keeping point source at the focus of the convex lens.
The light from the sun reaches the earth in the form of plane wavefronts.


3. Cylindrical wavefront : A wavefront originating from a linear source of light is called cylindrical wavefront.
Wave normal :
A normal or perpendicular drawn to the surface of a wavefront at any point, in the direction of propagation of light, is called a wavenormal.
The wavenormal gives the direction of propagation of light.