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

Options

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

B

Explanation

Given enengy flux `phi = 20 W//cm^2`
Area ` A = 30 cm^2`
total time ` t= 30 ` minutes `= 30 xx 60 =1800 s`
total energy incident on the surface in time t is
` E= phi At = 20 xx 30xx 1800 J`
`= 108 xx 10^4 J`
Momentum of the incident light is
`p=(E )/(c ) = ( 108 xx 10^4 )/( 3 xx 10^8)`
`= 36 xx 10^4 kg ms^(-1)`
No light is reflected from the surface . therefore momentum of the reflected light will be zero
hence , momentum delivered to the surface
`= 36 xx 10^(-4) kg ms^(-1)`