Question

If two slits in Young's experiment are $0.4\,mm$ apart and fringe width on a screen $200\,cm$ away is $2\,mm$ the wavelength of light illuminating the slits is

• A. 500 nm
• B. 600 nm
• C. 400 nm
• D. 300 nm

Answer 1

Answer: (C)

400 nm

Answer 2

Fringe width $W=\frac{D \lambda}{d}$
Given. $d=0.4\, mm =0.04\, cm,\, D=200\, cm$
$W=2\, mm=0.2\, cm$
$\therefore 0.2=\frac{200 \times \lambda}{0.04}$
$\Rightarrow \lambda=400\, nm$