Question

In a double-slit experiment, the distance between the slits is $d$. The screen is at a distance $D$ from the slits. If a bright fringe is formed opposite to a slit on the screen, the order of the fringe is

• A. $\frac{d^2}{2\lambda D}$
• B. $\frac{d}{2\lambda D}$
• C. $\frac{d^2}{4\lambda D}$
• D. $0$

Answer 1

Answer: (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, $x=\frac{n\lambda D}{d}=\frac{d}{2}\Rightarrow n=\frac{d^2}{2\lambda D}$