Question

A double slit, $S_1-S_2$ is illuminated by a light source $S$ emitting light of wavelength $\lambda$. The slits are separated by a distance $d$. A plane mirror is placed at a distance $D$ in front of the slits and a screen is placed at a distance $2D$ behind the slits. The screen receives light reflected only by the plane mirror. The fringe-width of the interference pattern on the screen is

• A. $\frac{D \lambda}{d}$
• B. $\frac{2 D \lambda}{d}$
• C. $\frac{3 D \lambda}{d}$
• D. $\frac{4 D \lambda}{d}$

Answer 1

Answer: (D)

Due to reflection virtual source will be formed at distance $D$ from mirror.
The effective distance of the screen $=2 D +2 D=4 D$
$\therefore$ Fringe width $=\frac{4 D \lambda}{d}$