First slide

Young's double slit Experiment

Question

In a Young's double slit experiment, 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 (centre of the screen)

Difficult

A

remains constant
B

keeps on decreasing
C

first decreases and then increases
D

first decreases and then becomes constant

Solution

$I_{p} = \frac{I_{max}}{2} [1 + cosϕ] = \frac{I_{max}}{2} (1 + \cos \frac{2 πy}{β})$
Where $β = \frac{Dλ}{d}$
First maxima is observed at P i.e., $\cos \frac{2 πy}{β} = 1; as D$ increases D will increase, the value of $\cos \frac{2 πy}{β}$ should be negative hence the ratio $I_{P} / I_{max}$ starts decreasing but starts increasing again as $\cos \frac{2 πy}{β}$ again starts becoming positive.

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