Monochromatic light is passed through a double slits, with separation between the slits, 0.5 mm and interference pattern is observed on a screen 2.5 m away. The first bright fringe is formed 2 mm away from central fringe. What is the wavelength of light?

400 nm

The correct answer is Option (3) → 400 nm

The position of n-th bright fringe,

$y_n=\frac{nλD}{d}$

for $n=1$,

$y_1=\frac{1×λD}{d}$

$⇒λ=\frac{y_1D}{D}=\frac{(2×10^{-3})(0.5×10^{-3})}{2.5}$

$=400nm$