Question

Light with an energy flux` 20 W//cm^2` falls on a non-reflecting surface at normal incidence. If the surface has an area of `30 cm^2`. the total momentum delivered ( for complete absorption)during 30 minutes is
A. `36xx10^-5Kg m//s`
B. `36xx10^-4Kg m//s`
C. `108xx10^4Kg m//s`
D. `1.08xx10^7Kg m//s`

Answer 1

Correct Answer - b
Here, energy flux,`phi=20W//cm^2 , Area, A=30 cm^2 , time t=30min=30xx60s`
Total energy falling on the surface in time t is, `U=phi A t=20xx30xx(30xx60)J`
Momentum of the incident light `=U/c=(20xx30xx(30xx60))/(3xx10^8)=36xx10^-4kgms^-1`
Momentum of the reflected light =0
`:.` Momentum delivered to the surface `=36xx10^-4-0=36xx10^-4kg m//s`