Question

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.

Answer 1

Total energy falling on the surface,
`U=18xx20xx30xx60J=6.48xx10^5J`
Total momentum delivered to the surface is
`P=U/c=(6.48xx10^5)/(3xx10^8)=2.16xx10^-3kgms^-1`
The average force exerted on the surface is
`F=p/t=(2.16xx10^-3)/(30xx60)=1.2xx10^-6N`
If the surface is a perfect reflector, the change
of momentum will be `=p-(-p)`
`=2p=2xx2.16xx10^-3kgms^-1`
Now average force,
`F=(2xx2.16xx10^-3)/(30xx60)=2.4xx10^-6N`