# Huygens’ Principle

Huygens’ postulated that light is a wave, which travels through a hypothetical medium called ether. This hypothetical medium has the strange property of occupying all space, including vacuum. The vibrations from the source of light propagate in the form of waves and the energy carried by them is distributed equally in all directions.

The concept of wavefront is central Huygens’ principle. A point source of light emits spherical wavefronts. A line source of light emits cylindrical wavefronts.

The line perpendicular to the wavefront at a point represents the direction of motion of the wavefront at that point. This line is called the ray of light and a collection of such rays is called a beam of light.When the source of light is at a large distance, any small portion of the wavefront can be considered to be a plane wavefront.

The Huygens’ principle states that

• Each point on a wavefront becomes a source of secondary disturbance which spreads out in the medium.
• The position of wavefront at any later instant may be obtained by drawing a forward common envelop to all these secondary wavelets at that instant.
• In an isotropic medium, the energy carried by waves is transmitted equally in all directions.
• If the initial shape, position, the direction of motion and the speed of the wavefront is known, its position at a later instant can be ascertained by geometrical construction.

To visualize Huygens’ construction, you can imagine a point source at the centre of a hollow sphere. The outer surface of this sphere acts as a primary wavefront. If this sphere is enclosed by another hollow sphere of larger radius, the outer surface of the second hollow sphere will act as a secondary wavefront. If the second sphere is further enclosed by another sphere of still bigger radius, the surface of the outermost (third) sphere becomes secondary wavefront and the middle (second) sphere acts as the primary wavefront. In two dimensions, the primary and secondary wavefronts appear as concentric circles.