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 = 20 W//cm^(2)`
Area, `A = 30 cm^(2)`
Time, `t = 30` min = `30 xx 60 s`
Now, total energy following on the surface in time t is,
`u = phi At = 20 xx 30 xx (30 xx 60) J`
Momentum of the incident light `= U/c`
`= (20 xx 30 xx (30 xx 60))/(3 xx 10^(8)) = 36 xx 10^(-4) kg-m//s`
Momentum of the reflected light `= 0`
`:.` Momentum delivered to the surface
`= 36 xx 10^(-4) -0 = 36 xx 10^(-4) kg-m//s`