A radiation of wavelength 200nm is propagating in the form of a parallel surface. The intensity of the beam is 5mW and its cross-sectional area is 1.0`mm^(2)`. Find the pressure exerted by radiation on the metallic surface if the radiation is completely reflected.

Explanation

`:.E=12400/lambda=12400/200=6.2eV~~10^(-18)J`
Number of photons passing a point per second is `n=P/E=(5xx10^(-9))/10^(-18)=5xx10^(9)`.momentum of each photon `p=E/C=3.3xx10^(-27)J//s`. Change in momentum after each stick `=2p=6.6xx10^(-27)J//s`
Total momentum change per second is `F=(dp)/(dt)=(nxx2p)/t=5xx10^(9)xx6.6xx10^(-27)=33xx10^(-18)N`
`:.pressure F/A=33xx10^(-12)N/m^(2)`