Question

State Huyghens principle, why does the backward envelope of the secondary brave front not exist ?​

Answer 1

Huygen's principle states that every point on a wavefront may be considered as a source of secondary waves and these points expand in every direction depending on their velocity.

According to Huygen's principle, the new wavefront is the forward envelope of the secondary wavelets. A backward envelope does not exist because the flow of energy is not backward. Every form of energy for example- light, wind, hydel energy, electricity moves forward from a region of lower potential to the region of higher potential. The wave which is also a form of energy hence moves forward from lower potential to higher potential and forms a forward envelope.