Question

In double-slit experiment, the slits are separated by a distance $d$ and the screen is at a distance $D$ from the slits. If a maximum is formed just opposite to each slit, then what is the order of the fringe so formed?

• A. $\frac{d^2}{2\lambda D}$
• B. $\frac{2d^2}{\lambda D}$
• C. $\frac{d^2}{\lambda D}$
• D. $\frac{d^2}{4\lambda D}$

Answer 1

The correct option is A $\frac{d^2}{2\lambda D}$

Constructive interference has occurred in front of one slit, its distance from centre is $x=\frac{d}{2}$

For bright fringes, $x=\frac{n\lambda D}{d}=\frac{d}{2}\Rightarrow n=\frac{d^2}{2\lambda D}$