Question

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

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

Answer 1

The correct option is C first decreases and then increases

Intensity at point P: $I=4I_0{\cos }^2\frac{\delta }{2}$
where $\delta =\frac{2\pi }{\lambda }\Delta$
and $\Delta =d\sin \theta =\frac{d{y}_n}{D}$
initially, first maxima is observed at P. Hence, $\delta =2\pi$
After that, when screen is moved away from the plane of the slits, D increases, condition for 2nd minima will be met. Hence, intensity decreases first and then increases.