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. `36 xx 10^(-5) kg-m//s`
B. `36 xx 10^(-4) kg-m//s`
C. `108 xx 10^(4) kg-m//s`
D. `1.08 xx 10^(7) kg-m//s`

Answer 1

Correct Answer - B
Given energy flux `phi =20W//cm^(2)`
Area `A = 30cm^(2)`
Time , `t= 30min =30 xx 60s`
Now , total energy falling on the surface in time is `u=phiAt =20 xx 30 xx (30 xx 60)J`
Momentum of the incident light `=U/C`
`=(20 xx 30 xx (30 xx 60))/(3xx 10^(8)) rArr = 36 xx 10^(-4) kg-ms^(-1)`
Momentun of the reflected light =0
`:.` Momentum delivered to the suface
`=36 xx 10^(-4) -0 =36 xx 10^(-4) kg-ms^(-1)`