Question

In YDSE the first bright fringe after the centre fringe is observed at P. If the screen is continuously moved away from plane of slits, then ratio of intensity at point P to intensity at point O [center of screen]

• A. remains same
• B. keeps on increasing
• C. can become $\infty$
• D. first decreases and then increases

Answer 1

The correct option is D first decreases and then increases

Path difference at P $=\frac{yd}{D}$.
As D increases, the path difference decreases. As the path length decreases from $\lambda$ to $\lambda /2$, the intensity at P decreases from maximum to minimum. However, as the path length decreases from $\lambda /2$ to 0, the intensity at P increases from minimum to maximum again.