KVPY SX SB Exam 20141 Question Paper

Question : 98 of 120

Marks: +1, -0

A 1.5 kW (kilo-watt) laser beam of wavelength 6400 Å is used to levitate a thin aluminium disk of same area as the cross section of the beam. The laser light is reflected by the aluminium disk without any absorption. The mass of the foil is close to

10 $^{-9}$ kg
10 $^{-3}$ kg
10 $^{-4}$ kg
10 $^{-6}$ kg

Validate

Solution:

Power of light = P

Force acting on Al disc

= {2P}/C

={2× 1.5× 10^3}/{3.0 ×10^8}

=10^{–5}

Force acting on Al disc = mg

10^{–5} = m × 10

m = 10^{–6}\ kg

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