<P> Electric discharge in gases occurs when electric current flows through a gaseous medium due to ionization of the gas . Depending on several factors, the discharge may radiate visible light . The properties of electric discharges in gases are studied in connection with design of lighting sources and in the design of high voltage electrical equipment . </P> <P> In cold cathode tubes, the electric discharge in gas has three regions, with distinct current - voltage characteristics: </P>

Why does a gas show a colour only when electricity is passed through the discharge tube