Light with an energy flux of `18 W//cm^(2)` falls on a non-reflacting 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

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