Question

Monochromatic light is passed through double slit and interference pattern is observed on screen $2.5\, m$ away. The separation between slits is $0.5 \,mm$. The first bright fringe is $3.5\, mm$ away from central fringe. The wavelength of light used is

• A. 600 nm
• B. 500 nm
• C. 650 nm
• D. 700 nm

Answer 1

Answer: (D)

$y=\frac{\lambda D}{d}$
Given $y=3.5 \,mm$
$3.5 \times 10^{-3}=\frac{2.5 \times \lambda}{0.5 \times 10^{-3}}$
$\therefore \lambda=\frac{3.5 \times 10^{-3} \times 0.5 \times 10^{-3}}{2.5}$
$=700 \,nm$