Question

The fluorescence of glass discharge tube at very low pressure is characteristic of
(A) The phosphorus in the material of the glass is
(B) The gas used in the tube
(C) The cathode
(D) All of the above

Answer 1

Hint When the gas in the tube is heated the electron gets excited and when it returns back to its ground state it loses energy which falls in the visible light region causing the fluorescence observed. It is the gas that glows and provides the light in the tube. Energy absorption happens in the form of electricity and remission in the form of light.

Complete step by step solution The florescence of the glass discharge tube is due to conversion of electric energy to light energy. When the gas used in the glass discharge tube, which is generally a halogen (except Br) is ionized or heated, its electrons absorb the energy to reach a higher energy state. When these electrons emit energy to reach a stable ground state, they release energy in the form of light of their respective wavelength. This is the reason for the illumination of the glass discharge tube at low pressure.

Therefore, the correct answer is option B.

Note: A discharge tube is a glass tube which has two electrodes. They are sealed and when a voltage is applied the pressure of the gas is reduced and a pressure is reached at which the current flows and the gas begins to glow. The illumination produced by the phosphorus present in the surface is called phosphorescent. Generally, only inert gasses are used in the tube because of their stable behavior.