Question

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. 0.15 cm
• B. 0.10 cm
• C. 0.25 cm
• D. 0.20 cm

Answer 1

The correct option is A 0.15 cm

$f=D=60cm$

For first minima,

$y=\frac{\lambda D}{a}=\frac{5\times {10}^{-7}\times 60}{2\times {10}^{-2}\times {10}^{-2}}=0.15cm$