Practice portal › Dual Nature of Matter and Radiation › Electron Emission and Cathode Rays
Asked in AIPMT 2008 · Discharge through gases
At low pressure the electrons leaving the cathode travel a long way before meeting anything, so the field accelerates them to high energy.
When such an electron strikes a gas atom, it hands over energy and lifts an atomic electron to a higher level.
The excited atom drops back almost at once and emits a photon, and the colour of that photon is set by the gas.
So the glow is a result of collisions between the charged particles emitted from the cathode and the atoms of the gas.
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