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When light goes through air we see the Beam (if we are using a laser beam) because of dispersion by dust. I have two questions:

What happens in pure air or gases and what happens in pure water? Why do we see the light if there is no dust?

Any mathematical reasoning or link will be welcome.

1 Answers1

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Any irregularity in the path of the bundle will scatter light. Also the molecules constituting the medium, air water or glass, exhibit Rayleigh scattering. A very intense bundle will cause visible Rayleigh scattering. See also What makes some laser beams visible and other laser beams invisible?

For mathematical formulae check here.

my2cts
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  • That's related to absortion of light in QM or is a different phenomena? –  May 22 '18 at 23:39
  • It is related. The scattering happens because the light polarises the molecules. Molecular polarisation is related to molecular quantum transitions, hence to absorption. – my2cts May 22 '18 at 23:51