Question

In the phenomenon of electric discharge through gases at low pressure, the coloured glow in the tube appears as a result of:
A. Collisions between the charged particles emitted from the cathode and the atoms of the gas.
B.Collisions between different electrons of the atoms of the gas.
C.Excitation of electrons in the atoms.
D.Collision between the atoms of the gas.

Answer 1

We know that Cathode rays are nothing but the electron beam travelling from negatively charged electrode (cathode) to positively charged electrode (anode) in discharge tube. In the travelling path there should be no obstacles. If there are any obstacles in the way which electrons are travelling, electrons cannot reach the anode.

Complete step by step answer:
In case of photoelectric effect the cathode is hit by a beam of photons which causes electrons in the cathode to come out of the cathode and go to anode. Potential difference is maintained between cathode and anode. Now gas will be present in that discharge tube. Now at constant temperature and volume if pressure is increased then the number of moles get increased which in turn increases the number of atoms i.e more obstacles will be present for the electron path So lower pressures are always favored for the production of cathode rays.
Collisions between charged particles emitted from cathode and atoms of gas. The discharge of electricity through liquefied gases is an interesting phenomenon which can be systematically studied with the help of a discharge tube. In discharge tube collision between charged particles emitted from cathode and atoms of the gas results in colorless glow in the tube.

Therefore, correct answer is option C.

Note:
Remember that there is no fixed value for pressure in the discharge tube. Generally low pressures are always preferred. Hence if there is another option which is lower than $0.001mm$ Hg that would have become the answer for this question.