What Are The Assumption Of Huygen's Principle ?
The assumptions are:
1. Every point on the given wavefront called 'primary wavefront' acts as a fresh source of new disturbance, called 'secondary wavelets' that travel in all directions with the velocity of light in the medium.
2. A surface touching these secondary wavelets tangentially in the forward direction at any instant gives a new wavefront at that instant. This is the secondary wave front.
This principle can be used to derive the laws of reflection and refraction.
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Take points 1,2,3 on the given wavefront AB. The distance traveled by light in t secs will be 'ct' where c is the velocity of light. With point 1 as centre and ct as radius, draw a sphere as shown (dotted circles). Repeat for points 2 and 3 and so on. Now draw a surface A1B1 and A2B2 such that they touch the sphere tangentially. Since the light travels in forward direction, the wave front A1B1 is the newly formed secondary wave front.
From Huygen's construction we learn that while the medium is homogeneous (i.e., optical properties of the medium are same everywhere) light moves forward and does not reflect back. We also assume that the secondary wavelets are emitted only in forward direction i.e., in the direction of advancement of the wave. If there is a sudden obstruction or charge in medium as discussed below, the wave may be reflected from the discontinuity. The secondary wavelets on the backward side should also be considered.