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 beam 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.

Dear Ria ,

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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