Light with an energy flux of `18 W//cm^2` falls on a non-reflecting surface at normal incidence. If the surface has an area of `20 cm^2`, find the average force exerted on the surface during a `30` minute time span.

Explanation

Total energy falling on the surface
`u = (18 Wcm^(-2))(30 xx 60s) (20 cm^(2)) = 6.48 xx 10^(5)]`
Therefore, the total momentum delivered is,
`rho = (u)/(c) = (6.48 xx 10^(5))/(3.0 xx 10^(8)) kg ms^(-1) = 2.16 xx 10^(-3) kgms^(-1)`
The average force exerted on the surface is,
`F = (rho)/(t) = (2.16 xx 10^(-3))/(30 xx 60)` [`:' F.t =` change in momentum]
`= 1.2 xx 10^(-6) N`