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Light with an energy flux of 18 W//cm^(2...

Light with an energy flux of `18 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^(-5)` kg m/s
(b) `36xx10^(-4)` kg m/s
(c) `108xx10^(4)` kg m/s
(d) `1.08xx10^(7)` kg m/s

A

`36xx10^(-5)` kg m/s

B

`36xx10^(-4)` kg m/s

C

`108xx10^(4)` kg m/s

D

`1.08xx10^(7)` kg m/s

Text Solution

Verified by Experts

The correct Answer is:
B
Intensity `I=(E )/(At)`
`therefore E=IAt`
`=(20W)/(cm^(2))xx 30 cm^(2)xx30xx60 sec`
`=108xx10^(4)` J
Final momentum `P_(f)=(U)/(C )=(108xx10^(+4))/(3xx10^(8))`
`therefore P_(f)=36xx10^(-4)Ns`
Initial momentum `P_(i)=0`
`therefore` Momentum imparted to wall,
`Delta P =P_(f)-P_(i)`
`=36xx10^(-4)-0`
`=36xx10^(-4)` kg m/s.
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