- Heat a filament by running electrons through it.
up. Typical temperatures are around 5000K. So the radiation peaks
What kind of spectrum do you expect?
A Planck curve.
- 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.
- Don't emit in the IR (``cool'' and energy
- 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
- 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.
Thu Jan 15 13:12:09 PST 1998