Fluorescence in Threlkeld

The Mining Museum in Threlkeld is packed with wonderful information. I particularly loved this fluorescence exhibit. Atoms contain electron energy levels. Energy reaches the electrons in the lowest energy levels in the form of light photons. The energy is calculated from the frequency by E=hf. Ultra-violet is high frequency so each photon carries a lot of energy. It can skip over several energy levels. What goes up must come down, the electron falls back to its ground state. But it doesn't have to do it in one jump. It can bump back down the intervening levels in several jumps. As it does so, it releases photons of light. If it bumps down in the smaller jumps, the photons have a lower frequency - low enough a frequency to be visible to our eyes. This is actually how fluorescent tubes work - but more on that another time.