Why does phosphorus glow?

Answer 1

The electrons of Phosphorus (and all elements) have energy levels in which their electrons can exist. The exact value of these energy level is determined by the magnitude of the positive charge of the elements nucleus and the wave nature of electrons themselves. These energy levels are numerous and exist even though there are not enough electrons to fill them. An electron may jump between any two of these energy levels (provided that the destination level is vacant) and in doing so either absorbs or emits a photon of light. The colour (wavelength, frequency) of this light depends on the difference of energy between the two energy levels traversed. An electron going from an inner orbit to a higher orbit absorbs a photon of light unique to the energy difference of those two orbitals, and an electron going from a higher orbit to a lower orbit emits a photon of light unique to the energy difference of those two orbitals.

Now the energy levels in phosphorus are such that there is a series of them that absorb certain frequencies of infrared light until the electron has absorbed a sum of energies that is equal to an energy difference that corresponds to a frequency of visible light. This series of energy levels is known as a 'ladder' as it provides a number of small steps by which a great 'height' can be reached. Once the electron has reached this 'height' then it can drop back down to it original ground state and emit the unique frequency of visible light we associate with phosphorus glow.

So in effect phosphorus is converting heat into light. The heat being low level Infrared light into visible light.

Now all elements have this effect. It is just that phosphorus' particular light emission is in the visible range.