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In the phenomenon of electric discharge through gases at low pressure, the coloured glow in the tube appears as a result of:

1. excitation of electrons in the atoms
2. collision between the atoms of the gas
3. collisions between the charged particles emitted from the cathode and the atoms of the gas
4. collision between different electrons of the atoms of the gas

Correct answer: collisions between the charged particles emitted from the cathode and the atoms of the gas

Solution

The coloured glow in the discharge tube is due to collisions between the charged particles (electrons) emitted from the cathode and the atoms of the gas, which excite the gas atoms. When these excited atoms return to their ground state, they emit light of specific wavelengths, producing the glow.

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