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

Here if momentum gained perpendicularly by given surface is p in time t then force exerted perpendicularly to the surface will be,
`F=(p)/(t) " "` ….(1)
For given perfectly non - reflecting surface (or completely absorbing surface)
`p=(U)/(c )` and so,
`F=(U)/(ct) " "` ...(2)
If intensity (or energy flux) of given radiation is I then,
`I=(U)/(At)rArr U=IAt " "` ....(3)
From equation (2) and (3),
`F=(IAt)/(ct)`
`therefore F=(IA)/(c )`
`therefore F=((18xx10^(4))(20xx10^(-4)))/((3xx10^(8)))`
`therefore F=1.2xx10^(-6)N`
Now, as per the question asked lastly in the statement if given surface is perfect reflector (i.e. incident energy is not absorbed at all) then momentum gained perpendicularly by the surface will be `p.=(2U)/(c )` and so,
`F.=2F=2.4xx10^(-6)N`