A beam of light of wavelength 590 nm is focussed by a converging lens of diameter 10 cm at a distance of 20 cm from it. Find the diameter of the disc image formed [UK PMT 2015]

Given, wavelength, $\lambda$ = 590 nm = 590 x 10^-9 m
Diameter of lens = 10 X 10^-2 m
$\therefore$  Radius of lens = 5 x 10^-2 m
Focal length of lens = 20 x 10^-2 m
Now, the diameter of image disc

$D=2R=2\left(1.22\frac{\lambda D}{b}\right)=\left(\frac{1.22\times 590\times {10}^{-9}\times 20\times {10}^{-2}}{5\times {10}^{-2}}\right)\times 2$

$\simeq 5.76\times {10}^{-4} \mathrm{cm}$