One property of light is that it spreads in the medium. This is because light has got some wave nature. We can understand this very well from the Huygens Principle.
WAVEFRONT
The continuous locus of all the particles of the medium which are vibrating in the same phase is called the wavefront.
The line perpendicular to the wavefront gives the direction of propagation of the wave or energy which is called rays.
NOTE:-
A large distance from the sources, the rays are nearly parallel and the wavefront is then approx plane.
HUYGENS PRINCIPLE
Huygen’s principle is a geometrical construction for determining the position of a new wavefront at some instant from the position of the previous wavefront.
It states that
(a) Every point on a given wavefront act as a source of secondary wavelets (called secondary waves) which travel in all directions with the velocity of light in the medium.
(b) The position of the new wavefront at any instant is a surface touching these secondary wavelets tangentially in the forward direction at that instant.
NOTE:-
There is no backward flow of energy during the propagation of a wave.
(Huygens argued that the amplitude of the secondary wavelets is maximum in the forward direction and zero in the backward direction).
| Introduction | Huygens Principle |
| Refraction and Reflection of Plane waves using Huygens Principle | Coherent and Incoherent Addition of Waves |
| Interference of Light Waves and Young’s Experiment | Diffraction |
| Polarisation | |
IMPORTANT QUESTIONS FOR PRACTICE
1. What is the shape of the wavefront in each of the following cases:
(a) Light diverging from a point source.
(b) Light emerging out of a convex lens when a point source is placed at its focus.
(c) The portion of the wavefront of the light from a distant star intercepted by the earth.