In Young's double-slit experment, the frist maxima is observed at a fixed point P on the screen. Now, the screen is continously moved away from the plane of slits. The ratio of intensity at point P to the intensity at point O (center of the screen)

Options

A. remains cosntant
B. keeps on decreasing
C. first decreases and then increases
D. first decreases and then becomes constant

Answer

c

Explanation

`I_(p) = (I_(max))/(2) [1 + cos phi] = (I_(max))/(2) (1 + cos.(2 pi y)/(beta))`
where `beta = D lambda // d`.
First maxima is observed at P, i.e., `cos.(2 pi y)/(beta)` should be negative. Hence, the ratio `I_(P) // I_(max)` starts decreasing but starts increasing again as `cos.(2 pi y)/(beta)` again starts becoming positive.