Question

Why there is no backward wavefront in Huygen's theory?

Answer 1

Huygen's theory:

1. All the points on a wavefront can be considered as new sources of wavefront themselves.
2. According to this principle in a homogeneous and isotropic medium, every point of a primary wavefront serves as the source of spherical secondary wavelets that travel with a speed and frequency equal to those of the primary wave.
3. The wavefront at some later instant of time is the envelope of these wavelets.
4. A wavefront is first generated by a source, which passes the wavefront creating more wavefronts.
5. In general, new wavefronts are propagated in the same direction as the parent wavefront.
6. This is because the flow of energy is in that direction, so the new wavefronts follow the energy flow.
7. Another reason why a wavefront can't travel backward is it creates destructive interference which destroys all wavefronts.

Hence, the wavefront can't travel backward.