Why do cathode rays give out light?

[mahela007]

Why do cathode rays illuminate the tube in which they are present? Also why do they stop giving out light at VERY low pressures?

[insane_alien]

the elctrons smash into atoms of gas that become excited due to the input of energy.

 

excited in this case means one or more of the electrons in the atom has jumped up to a higher orbital.

 

when the electron jumps back down to its usual orbital, a phoon of light is released. this is what you are seeing.

 

at very low pressures, the electron beam rarely hits a molecule so you don't see it giving out any light.

[mahela007]

but that doesn't agree with this observation

"In 1838, Michael Faraday passed a current through a rarefied air filled glass tube and noticed a strange light arc with its beginning at the cathode (negative electrode) and its end almost at the anode (positive electrode). In 1857, German physicist and glassblower Heinrich Geissler sucked even more air out with an improved pump, to a pressure of around 10-3 atm and found that, instead of an arc, the glow filled the tub"- wikipedia

 

Why did Geissler, even with a much lower pressure , see the whole tube illuminate? With less molecules of air shouldn't the tube have become darker?

 

Ok.. I found the answer Via IRC

Edited July 1, 2009 by mahela007
SOLVED

[insane_alien]

yes this is because the excited atoms can travel a distance without colliding with other atoms which will cause the excited atoms to discharge a photon.

 

further reduce the pressure and the glow will be reduced.