A parallel be beam of radiation of intensity `10W` and of area of cross section `1cm^(2)` is falling on a plane surface at an angle `60^(@)` with normal to the surface. The surface is partially reflecting with reflection coefficient `0.5` and absobing the remaining. Choose the incorrect option of the following:

Options

A. Force on the surface normal to it is `2.5xx10^(-12)N`
B. Force on the surface parallel to it is `(2.5)/(sqrt(3))xx10^(-12)N`
C. Net force on the surface `=(5)/(sqrt(3))xx10^(-12)N`
D. Net force on the surface acts at an angle `60^(@)` with normal to the surface

Answer

D

Explanation

Force on the surface normal to it is
`F_(n)=(IA costheta)/(C )(1+r)=(10xx10^(-4)xx(1)/(2)(1.5))/(3xx10^(8))=2.5xx10^(-12)N`
force on the surface parallel to it is
`F_(t)=(IA sintheta)/(C )(1-r)=(10xx10^(-4)xx(sqrt(3))/(2)(0.5))/(3xx10^(8))=(2.5)/(sqrt(3))xx10^(-12)N`
Net force on the surface
`F=sqrt(F_(n)^(2)+F_(t)^(2))=(5)/(sqrt(3))xx10^(-12)N`
`Tan alpha=(F_(t))/(F_(n))=(1)/(sqrt(3))`
`implies alpha=30^(@)`