A parallel beam of monochromatic light of wavelength 500 nm is incident normally on a perfectly absorbing surface the power through any cross section of the beam is 10 w find
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(a) Energy of each photon is
E = hc / 
= 1242 eV – nm/ 500 nm
= 2.48 eV
In one second 10 J energy passes through any cross section of the beam,
Thus the no. of photons crossing a cross section is =
n = 10 J / 2.48 eV
n = 2.52 * 1019
(b)
total momentum of all the photons =
p = nh/  = nh / c = 10 J / 3*108
= 3.33 * 10 –8 N-sec
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