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. `36xx10^(-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
For complete absorption, `p = (U)/(c)`
where U = Energy carried by light `= IAt`
`:. p = (IA t)/(c)`
Here, `I = 20 W//cm^(2), A = 30 cm^(2), t = 30 min = 1800 s`
So, `p = (20 xx 30 xx 1800)/(3 xx 10^(8)) = 36 xx 10^(-4) kg m s^(-1)`