In the phenomenon of electric discharge through gases at

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

The coloured glow in the tube appears as a result of excitations of electrons in the atoms.