Question

A parallel monochromatic beam of light is incident normally on a narrow slit. A diffraction patten is formed on a screen placed perpendicular to the direction of incident beam. At the first maximum of the diffraction pattern the phase difference between the rays coming from the edges of the slit is
A. (a) `0`
B. (b) `pi/2`
C. (c) `pi`
D. (d) `2pi`

Answer 1

Correct Answer - D
The phase difference `(phi)` between the wavelets from the top edge and the bottom edge of the slit is `phi=(2pi)/(lambda) (d sin theta)` where d is the slit width. The first minima of the diffraction pattern occurs at `sin theta=lambda/d`
so `phi=(2pi)/(lambda)(dxx(lambda)/(d))=2pi`