A beam of light of wavelength 590 nm is focussed by a converging lens of diameter 10.0 cm at a distnce of 20 cm from it. find the diameter of he disc image formed.

Explanation

The angular radius of the central bright disc in a diffraction pattern from circular apeture is given by
`sintheta=(1.22lamda)/b`
`=(1.22xx590xx10^-9m)/(10.0xx10^-2m)=0.7xx10^-5rad`
The radius of the bright disc is
`=0.7xx10^-5xx20cm=1.4xx10^-4cm`
`The diameter of the disc image `=2.8xx10^-4cm`.