An impluse laser may be treaed as a source of phtons that are emitted during the time interval of the pulse which is followrd by a time interval when no photons are produced. Pulse are periodically repeated. A laser beam of diameter `d = 10` microns is directed upward and is perpendicular to the then foil surface which has an index of reflection` rho = 0.50` (see the sketch of the experiment). The index of reflection of the surface is the ratio of the relected energy of foil that can be supported in the air solely by the light pressure of the laser beam ?

Options

A. `lt 39 g`
B. `lt 3.1 xx 10^(-12) g`
C. `lt 39 mg`
D. `lt 3.7 g`

Answer

C

Explanation

Change in momentum (in reflection) `= (2rhoE)/(c)`
Change in momentum (in absorbtion) `= (1-rho) (E)/(c)`
`F = ((2rhoE)/(c) + (1-rho) E/c)/(t) rArr ((1+rho)E)/(tc) = Mg`
`M = ((1-rho)E)/(tcg) rArr ((1+05) xx 10)/((0.13 xx 10^(-3)) xx (3 xx 10^(8)) xx 10)`
`M = 38 xx 10^(-6) kg`
`M = 38 mg`