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 exrted on the surface during a 30 minute time span.

Explanation

The total energy falling on the surface is
`U=(18W//cm^(2))xx(20cm^(2))xx(30xx60)`
`=6.48xx10^(5)J`
Therefore, the total momentum delivered (for complete absorption) is
`p=(U)/(c)=(6.48xx10^(5)J)/(3xx10^(8)m//s)=2.16xx10^(-3)kgm//s`
The average force exerted on the surface is
`F=(p)/(t)=(2.16xx10^(-3))/(0.18xx10^(4))=1.2xx10^(-6)N`
How will your result be modified if the surface is a perfect reflector?