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 20cm² , find the average force exerted on the surface by the incident light.

Answer 1

Given 

Energy flux = 18Wcm^-2

\(=18\frac{J}{s-cm^2}\)

Therefore, energy falling on 20 cm² in one second is 

U = Flux x Area x Time

Hence, total momentum delivered (for complete absorption) on to 20 cm² area, in 1 second is

Therefore the average force exerted on the surface is: