Question

A linear aperture whose width is $0.02cm$ is placed immediately in front of a lens of focal length $60cm$. 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. $0.12cm$
• B. $0.15cm$
• C. $0.25cm$
• D. $0.20cm$

Answer 1

The correct option is B $0.15cm$

As given,
$f=D=60cm$

For first minima,
$y=\frac{\lambda D}{a}=\frac{5\times {10}^{-5}\times 60}{2\times {10}^{-2}}=0.15cm$$