Question

Two coherent points sources $S_1$ and $S_2$ vibrating in phase emit light of wavelength $\lambda$. The separation between them is $2\lambda$. The light is collected on a screen $\sum$ placed at a distance $D>>\lambda$ from the slit $S_1$ as shown, in the fig. Find the minimum distance so that intensity at $P$ is equal to intensity at $O$.

• A. $D$
• B. $D_{/3}$
• C. $\sqrt{3}D$
• D. $D_{/2}$

Answer 1

Answer: (C)

$\sqrt{3}D$

$S_1{S}_2\cos \theta =n\lambda$
$2\lambda \cos \theta =\lambda$
$\theta ={60}^0$
$x=D\tan {60}^0=\sqrt{3}D$.