A plane light wave of intensity `I=0.20Wcm^-2` falls on a plane mirror surface with reflection coefficient `rho=0.8`. The angle of incidence is `45^circ`. In terms of corpuscular theory, find the magnitude of the normal pressure exerted on that surface.

Explanation

Momentum corresponding to incident photons normal to the surface,
`((dp)/(dt))_(i nci dent)=(I)/(c )dAcos^(2)theta`
Since reflection coefficient is `rho`, so the momentum of the reflected photons per second normal to surface,
`((dp)/(dt))_(refl ected)=(I)/(c)dArhocos^(2)theta`
Hence, rate of change of momentum of the photons,
`((dp)/(dt))_(phot ons)=(I)/(c)dA(rho-1)cos^(2)theta`
From Newton's thirt law,
`F=((dp)/(dt))_(surface)=(I)/(c)dA(rho+1)cos^(2)theta`
Hence, pressure exerted on surface,
`P=(dF)/(dA)=(I)/(c)(rho+1)cos^(2)theta`
On substituting values, we get
`P=0.5Ncm^(-2)`