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 Rayleigh scattering 

There is a ""citation needed"" at the claim that Rayleigh scattering can be seen. I just did some research on Rayleigh scattering in air; I added some content to Rayleigh scattering. The bottom line is that at atmospheric pressure, Rayleigh scattering at 532 nm is about 10^-5 per meter. A 1 mW green laser has a luminous power of 0.6 lumen, and hence rayleigh scattering is 6e-6 lumen per meter over a 2 mm beam (a 0.002 m2 surface). The beam will hence appear roughly as if it were illuminated at 6e-6/0.002 = 3 millilux, which is comparable to a moonless clear night sky. (I should actually correct the number to deal with the angular distribution). A dark-adapted eye should be able to see this, especially when you look along the beam, which multiplies the apparent brightness by 1/sin(theta) where theta=0 deg means you look paralell to the beam.

Now I'm not sure how to turn this original research into verifiable encyclopedic content.  (t) "