Explain why a discharge tube appears dark when evacuated to a very low pressure.

In a discharge tube light is produced by the transitions of the excited gas atoms as they return to their ground states. The intensity of light emitted depends on the number of excited atoms. As the pressure is reduced, the density of gas decreases, and the electrons are able to travel from the cathode to the anode of the tube without undergoing appreciable collisions with the gas atoms. Hence, as the pressure of gas is decreased the number of excited atoms becomes so low that no appreciable light is emitted by the tube.

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