CBSE Class 11-science Answered

Discharge tube is a long glass tube having two metal plates, sealed at its two ends as electrodes. It has a side tube through which air can be pumped out by using a vacuum pump, so that experiments can be performed at low pressure. When the pressure in discharge tube is reduced to 10^-3 atm or 5 mm of Hg, a pink glow is emitted by the air inside the tube. Now, when the pressure of air in the discharge tube is reduced to about 10^-5 atm or 0.001 mm of Hg and a high voltage is applied to the electrodes, the emission of pink light by air stops. But the phenomenon of fluorescence is observed in which the glass walls of the discharge tube at the end opposite to the cathode start to glow with a greenish light and inside of the discharge tube turns dark. It is now deduced that some invisible rays were formed at the cathode, which on striking the glass tube emitted a green light. Since they are formed at the cathode they are known as cathode rays.