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. (a) remains constant
B. (b) keeps on decreasing
C. (c) first decreases and then increases
D. (d) first decreases and then becomes constant

Answer

C

Explanation

`I_p=(I_(max))/(2)[1+cosphi]=I_(max)/2(1+co s(2piy)/(beta))`,
Where `beta=(Dlambda)/(d)`
First maxima is observed at P i.e., `co s (2piy)/(beta)=1`, as D increases b will increase, the value of `co s(2piy)/(beta)` should be negative hence the ratio `I_P//I_(max)` starts decreasing but starts increasing again as `co s(2piy)/(beta)` again starts becoming positive.