Question

A slit of width 0.1 mm is illuminated normally d by light of $\lambda = 5000 \mathring A$. Diffraction bands are observed on a screen, 1 m away from the slit. Third dark band is at a distance x from the central maximum. x is approximately

• A. 5 mm
• B. 10 mm
• C. 15 mm
• D. 20 mm

Answer 1

Answer: (C)

15 mm

Answer 2

$n^{th}$ order minimum in diffraction is given by $Y_n = \frac{n \lambda D}{d}$
Here, $n = 3 , \lambda = 5000 \mathring A = 5 \times 10^{-7} m , D = 1 \: m \: d = 0.1 \: mm = 10^{-4} \: m , Y_3 = x = ?$
$\therefore \, \, \, Y_3 = x = \frac{3 \times 5 \times 10^{-7} \times 1}{10^{-4}} = 15 \: mm$