A linear aperture whose width is 0.02 cm is placed immediately in front of a lens of focal length 60 cm. The aperture is illuminated normally by a parallel beam of wavelength $5 \times 10^{-5}$ cm. The distance of the first dark band of the diffraction pattern from the centre of the screen is:

A 0.20 cm

B 0.15 cm

C 0.10 cm

D 0.25 cm

The answer is B.

Solution

$y = \lambda f/a$.

f = D = 60 cm
For the first minimum: $y = \frac{\lambda D}{a} = \frac{5\times10^{-7}\times60}{2\times10^{-2}\times10^{-2}} = 0.15$ cm

Concept single-slit-first-minimum-distance

40 questions from this chapter have appeared across 18 exam years.