Huygens’ Theory

The locus of all those particles which are vibrating in the same phase at any instant is called a wavefront; thus a wavefront is a surface of constant phase.

When the source of light is a point source, the wavefront which is a sphere with the centre as the source is called a spherical wavefront.

When the source of light is linear and all the points equidistant from the source lie on a cylinder, the wavefront which is cylindrical in shape is called a cylindrical wavefront.

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

The speed with which the wavefront moves outwards from the source is called the speed of wave.

The secondary light waves emitted in all directions by each point on a wavefront, which travel with the speed of light in the medium, are called wavelets.

Huygens' Principle states that:

• Each point on a wavefront acts as a secondary source of light emitting secondary light waves called wavelets in all directions.
• These wavelets travel with the speed of light in the medium.
• The new wavefront can be obtained by taking the envelope of these secondary wavelets travelling in the forward direction.
• The wavelets travelling in the backward direction are ineffective.

Additional Points:

• Huygens' principle is a geometrical construction which gives the shape of the wavefront at any time and allows us to determine the shape of the wavefront at a later time.
• It also tells how a wavefront propagates through a medium.
• The energy of the wave travels in a direction perpendicular to the wavefront.