[Phys-L] colors when electrons scatter from a gas

[Carl Mungan <mungan@usna.edu>]

Someone asked me what determines the colors when an electron beam scatters from gas atoms (an example would be the bluish light seen as the beam track in the introductory e/m experiment)?

The question specifically is why one sees only the 492 nm line (see https://physics.stackexchange.com/questions/79441/which-cyan-colored-line-is-produced-in-the-thomson-e-m-apparatus <https://physics.stackexchange.com/questions/79441/which-cyan-colored-line-is-produced-in-the-thomson-e-m-apparatus>) and not the full visible spectrum of helium that one sees from a helium discharge tube (done in a different introductory lab experiment)?

We suppose it has something to do with a trade-off involving gas density, state lifetimes, and the electron beam current.

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Carl E. Mungan, Professor of Physics  410-293-6680 (O) -3729 (F)
Naval Academy Stop 9c, 572C Holloway Rd, Annapolis MD 21402-1363 
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