Question

Light with an energy flux of `40W//cm^(2)` falls on a non-reflecting surface at normal incidence. If the surface has an area of `20cm^(2)`, find the average force exerted on the surface during a `30min` time span.

Answer 1

The energy incident on surface is
`U = Iat = 40 xx 10^(4) xx 20 xx 10^(-4) xx 30 xx 60`
`= 1.44 xx 10^(6)J`
Total momentum delivered (for completa absorption)
`p = (U)/(c ) = (1.44 xx 10^(6))/(3xx10^(8)) = 0.48 xx 10^(-2)kgm//sec`
The average force extered on surface
`F = (p)/(t) = (0.48xx10^(-2))/(30xx60) = 0.27 xx 10^(-5)N`