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  4. A 1.5 kW laser beam of wavelength 6400 mathringA is used to levitate a thin aluminium disc of same are a 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

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Q. A $1.5 \,kW$ laser beam of wavelength $6400\, \mathring{A}$ is used to levitate a thin aluminium disc of same are a 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

KVPYKVPY 2014Electromagnetic Waves

Solution:

Force due to radiation pressure $=$ Weight of disc
$\Rightarrow \frac{2p}{c} =mg$
$\Rightarrow \frac{2\times1.5\times10^{3}}{3\times10^{8}}=m\times10$
$\Rightarrow m=10^{-6} \,Kg$