There are several types of light bulbs and they
emit light through different mechanisms. I'll try to
explain how incandescent light bulbs work.
A regular incandescent light bulb relies on the
fact that all bodies with a temperature greater
than absolute zero emit radiation. This
radiation is logically named thermal radiation
and the intensity of radiation emitted at a given
wavelength is a function of the temperature of the
body. At low temperatures a body emits very low
intensity radiation, mostly of lower energy than
visible light. The visible light that is emitted is
of far to low intensity to be seen by human eyes.
However at high temperature the intensity of visible
light (and other radiation) emitted by the body increases.
Depending on its specific physical properties a body
may emit more or less visible light at a given temperature.
Incandescent light bulbs use a thin wire called a filament
(traditionally made of tungsten, perhaps currently made
of a different material) as a thermal radiation emitting
body. The filament has a very tiny axial diameter
(it is skinny) and therefor a fairly high resistance
to current. When AC current flows through the filament
it is heated very rapidly to a very high temperatures
(that is why incandescent light bulbs are hot) and it
emits lots of thermal radiation in the visible spectrum.
Answered by: Ed Nicholas, M.S., Medical Science Grad. Student
'Physics is mathematical not because we know so much about the physical world, but because we know so little; it is only its mathematical properties that we can discover.'