A sphere of radius 1.00 cm is placed in the path of a parallel beam of light of large aperture. The intensity of the light is 0.50 W/cm2. If the sphere completely absorbs the radiation falling on it, find the force exerted by the light beam on the sphere.

Question

Arun · Answer

Change in momentum of photon due to absorbtion = h /λ

no of photons per unit time = p/(hc/λ ) p = power of beam

therefore change in momentum per unit time = force = h /λ * p/(hc/λ ) = p /c

p = intensity * area = .5 * 4*pie*r² = .5 *4*3.14*(1)² = 6.28 W

so force = 6.28 /3 *10⁸ = 2.09 * 10^-8 N . The answer of this question in some books as found by me is incorrect

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A sphere of radius 1.00 cm is placed in the path of a parallel beam of light of large aperture. The intensity of the light is 0.50 W/cm2. If the sphere completely absorbs the radiation falling on it, find the force exerted by the light beam on the sphere.

A sphere of radius 1.00 cm is placed in the path of a parallel beam of light of large aperture. The intensity of the light is 0.50 W/cm2. If the sphere completely absorbs the radiation falling on it, find the force exerted by the light beam on the sphere.

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