How do Incandescent lights work?

• Heat a filament by running electrons through it. up. Typical temperatures are around 5000K. So the radiation peaks at:

  

• What kind of spectrum do you expect?

  A Planck curve.

How do fluorescent lights work?

• Turn on the power, start boiling electrons off the filaments and send them flying back and forth through the tube at 60Hz.

• The electrons crash into atoms, collisionally excite the electrons in the atoms to higher levels. When the electrons in the atoms drop back down to lower excitation states, photons are emitted - for most tubes, these are primarily UV photons.

• With no phosphor coating, you get a ``black light''.

• With a phosphor coating, the UV photons are absorbed by the atoms in the phosphor and excite them to high levels. The de-excitation occurs through a cascade of photons where there area number of visible light photons for each UV photon absorbed.

Fluorescence and fluorescent lights

• Don't emit in the IR (``cool'' and energy efficient).

• Do emit in the UV (this is why the shoulders in your clothes fade).

• Are better for plants as they mimic the solar spectral coverage better.

• ``Whiter-than-white'' detergents leave a residual of phosphor-like material on your clothes that absorb UV from the Sun and re-emit blue photons to make the balance of light reflected seem whiter

• The ``decay time'' for some of the downward atomic transistions can be quite long. Good thing or glow-in-the-dark toothbrushes would not be possible.

• The are many examples of fluorescence in astronomy. They tend to be the red glowing diffuse nebulae because of the Hydrogen emission line produced when drop from the second to the first excited state. The ``HII region''; illustrated below this is a ``radiationally excited'' example. Energetic photons (UV) from the hot stars at the center of the hydrogen cloud are absorbed by H atoms and either knock the electrons to high energy states or ionize the atoms. On de-excitation or recombinations, the electrons drop down an produce a cascade of lower energy photons. The ones in the visible part of the spectrum produce the glow we see.

  

• The Aurora of the Earth is an example of a ``collisionally'' excited gas that radiates.