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Name: Justin S.
Status: student
Age: 20s
Location: N/A
Country: N/A
Date: 2001-2002


Question:
When using the Leybold q/m Apparatus, why is the beam of electrons coming out of the electron gun blue?


Replies:
Justin,

I cannot say for certain, because I do not have the apparatus. I can however, think of two very likely possibilities.

One, of course, is that the source of electrons is also a source of blue light. To make a material emit electrons, it must receive extra energy. One way to do this is through heat. If a filament is heated to make it emit electrons, it may also be hot enough to glow blue.

A less likely possibility is electrons colliding with air molecules, and then blue photons being released during the collisions. For this to be true, there would have to be a huge number of electrons all losing the same amount of energy when crashing into air molecules.

One way to determine whether a blue-glowing filament is the cause is with a focusing mirror. Get a concave mirror with a focal length of several centimeters (8-12cm). Place the mirror at an angle of about 45 degrees in the beam, at least twice the focal length from where the electrons are produced. Hold up a paper in the reflected light, about two focal lengths from the mirror. If you can see a blue filament projected on the paper, you are seeing the source of the blue light. You may have to adjust the paper's location to refine the focus.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College


It sounds like emission of light from excitation of the gas in the apparatus by the electron beam, but I am not sure what gas is in the apparatus.

Vince Calder



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