Question

How did Huygens justify the absence of the backwave on a spherical wavefront?

Answer 1

Huygens argued that the amplitude of the secondary wavelets is not the same in all directions.

According to him, the amplitude of a secondary wavelet is proportional to `1/2(1 + cos theta)`, where θ is the angle between the direction of propagation and the normal to the wavelet.

In the forward direction,

θ = 0°

∴ cos 0° = 1

The factor is `1/2(1 + 1)` = 1 (Maximum)

In the backward direction,

θ = 180°

∴ cos 180° = −1

The factor is `1/2(1 - 1)` = 0 (Zero)

Hence, there is no backwave.