The glass becomes hot when an electric bulb is kept switched on. It is not so in the case of a tube light. Why?

When a current is passed through the filament of an electric bulb, the filament gets heated to very high temperature (about 3000 degrees centigrade) when it emits light according to the laws of black body radiation. Note that, in the electric bulb the power dissipated gets converted to both heat and light. In fact, only a small fraction of the power is available to us as light and the rest appears as heat, which heats up the glass bulb.

However, in a tube light, the electrons emitted by the filaments at the ends of the tube excite the gas atoms/molecules inside the tube through an electric discharge process. These excited molecules and atoms radiate UV radiation which cause fluorescence of the white phosphor coated on the inside wall of the glass tube. The light emitted is not due to black body radiation. Actually, the colour of light obtained from a tube light would correspond to a colour temperature of about 6,500 degrees Kelvin. This only represents the emission colour. It may be pointed out here that this process of fluorescent light emission does not require heat as input; therefore there is no heating of the surrounding glass tube.

Source: The Hindu