the glass becomes hot when an electric bulb is kept switched on.it is not so in the case of tube light. why?

Answer 1

An electric INCANDSCENT bulb uses a tungstene filament (twisted wire) that is heated by the current passing through to several hundred degrees. It is this heat that heats the glass. It is also the proof of inefficiency of the bulb, because it is transforming 80% of the energy it receives into heat, giving only 20% for light!
In the FLUORESCENT tubes, the light is produced by the ionisation of the internal gas (neon or argon). This ionisation is initiated by heating two small filaments at teach end of the tube, that heat up a small drop of mercury. Once the mercury is vaporised inside the tube, the "starter" opens, creating a high voltage puls across the whole tube, ionising the gas that then becomes conductor: a small current passes through the gas, and the filaments are disconnected. Since the ionised and conductive gas has nearly no resistance, the current is limited by the "ballast" (an inductor inserted in series).
Since the CURRENT is limited, there is no "Joule Effect" (heating) and the tube stays pretty cool.
Their efficiency is about 100 times more than a bulb... (in terms of power/light ratio).

Answer 2

The incandescent light bulb or incandescent lamp is a source of artificial light that works by incandescence. An electric current passes through a thin filament ( tungesten), heating it and causing it to emit light. The enclosing glass bulb prevents the oxygen in air from reaching the hot filament, which would be otherwise rapidly destroyed by oxidation. hence the glass of a buld becomes hot . whereas

(tube light) The main principle of fluorescent tube operation is based around inelastic scattering of electrons. An incident electron (emitted from the coils of wire forming the cathode electrode) collides with an atom in the gas (such as mercury, argon or krypton) used as the ultraviolet emitter. This causes an electron in the atom to temporarily jump up to a higher energy level to absorb some, or all, of the kinetic energy delivered by the colliding electron.This higher energy state is unstable, and the atom will emit a photon as the atom's electron reverts to a lower, more stable, energy level. The photons that are released from the chosen gas mixtures tend to have a wavelength in the ultra-violet part of the spectrum. This is not visible to the human eye, so must be converted into visible light. This is done by making use of fluorescence. This fluorescent conversion occurs in the phosphor coating on the inner surface of the fluorescent tube, where the ultra-violet photons are absorbed by electrons in the phosphor's atoms, causing a similar energy jump, then drop, with emission of a further photon. The photon that is emitted from this second interaction has a lower energy than the one that caused it. The chemicals that make up the phosphor are specially chosen so that these emitted photons are at wavelengths visible to the human eye. and as there is no heating occurs the tubelight is not hot compared to the bulb . but the temperature is definetly higher than the room temperatue.

Answer 3

I think it is because tube light uses gas (neon?) instead of a piece of metal wire.