A radiation of wavelength 200nm is propagating in the form of a parallel surface. The power 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)/(lamda)=(12400)/(2000)=6.2eV~~10^(-18)J`
Number of photons passing a point per
second is `n=(P)/(E)=(5xx10^(-3))/(10^(-18))=5xx10^(15)`
momentum of each photon `p=(E)/(C)=3.3xx10^(-27)J//s`
Change in momentum after each strike
`=2p=6.6xx10^(-27)J//s`
Total momentum change per second is
`F=(dp)/(dt)=(nxx2p)/(t)=5xx10^(15)xx6.6xx10^(-27)`
`=33xx10^(-12)N`
`:.` pressure `=(F)/(A)=33xx10^(-6)N//m^(2)`