Assertion : Contribution of the wavelets lhying on back on a wavefront is zero.
Reason : The contribution of a wavelet in any direction making angle `theta` with the wavelet is proportional to `1/2( 1 + cos theta)`. In the case, `theta= 180^@`.

Options

A. Both A and R are true and R is the correct explanation of A
B. Both A and R are true but R is NOT the correct explanation of A
C. A is true but R is false
D. A is false and R is true

Answer

A

Explanation

According to Huygens theory each and every point on a wavefront is the source of secondary wavelets. Secondary wavelets do not proceed backward. So the assertion is true.
Kirchhoff.s explained that am-in.de of secondary wavelets is proportional to `(1+ cos theta)` , where e is the angle between the ray at the point of consideration and direction of secondary wavelets. In the backward direction `theta = 180^@` , so `1 + cos theta = 0` , so there secondary wavelets do not proceed backward. Hence assertion and both are true and the reason properly explains the assertion.