Question

Please Help! When does the double-slit interference have its first dark spot on either side of the central peak intensity?

Answer 1

A, C, and D are true when a light spot appears, so those are false.

The answer is therefore B.

Answer 2

Explanation :

In double slit experiment, the consecutive dark and bright fringes are formed in the screen.

The condition for bright fringes is :

$dsin\theta=n\lambda$

and

The condition for dark fringe is :

$dsin\theta=(n+\dfrac{1}{2})\lambda$

So, the first dark spot on either side of the central peak intensity is observed when the when the paths d₁ and d₂ differ by half a wavelength.  

Hence, the correct option is (B).