A 1.5 ,kW laser beam of wavelength 6400 ,math | vedclass

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(D) For a perfectly reflecting surface,the radiation force $F$ exerted by a beam of power $P$ is given by $F = \frac{2P}{c}$,where $c$ is the speed of

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$10^{-6}$

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(D) For a perfectly reflecting surface,the radiation force $F$ exerted by a beam of power $P$ is given by $F = \frac{2P}{c}$,where $c$ is the speed of light.For the disc to levitate,this radiation force must balance the gravitational force (weight) acting on the disc: $F = mg$.Equating the two,we get $\frac{2P}{c} = mg$.Given: $P = 1.5 \,kW = 1.5 	imes 10^3 \,W$,$c = 3 	imes 10^8 \,m/s$,and taking $g = 10 \,m/s^2$.Substituting the values: $\frac{2 	imes 1.5 	imes 10^3}{3 	imes 10^8} = m 	imes 10$.$\frac{3 	imes 10^3}{3 	imes 10^8} = 10m$.$10^{-5} = 10m$.$m = 10^{-6} \,kg$.

$A$ $1.5 \,kW$ laser beam of wavelength $6400 \,\mathring A$ is used to levitate a thin aluminium disc of the same area as the cross-section of the beam. The laser light is reflected by the aluminium disc without any absorption. The mass of the foil is close to ......... $kg$.

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