One of my friends had her class do some genetic engineering on bacteria. They used small loops of DNA called plasmids to insert genes from jellyfish into the bacteria. The genes let the bacteria make a protein that absorbs ultraviolet light and re-emit a greenish light. The bacteria were grown in agar plates. Here is a photograph of some of them taken in ultraviolet light.
The physics of this trick with light is explained like this:
1. Remember that elecrons are found on different energy levels in an atom. This is the "electron orbits" model that is used in GCSE Chemistry.
2. In A Level Physics, we cut out a section of the energy levels and concentrate on that.
3. Here are the cut out energy levels. What happens now is that a photon of ultraviolet light comes in from the left. This high energy light provides energy to make the electron go up several levels. The electron could fall back all the way in one go and release a similar ultraviolet photon, or it could fall down the levels in stages as shown. These smaller falls release smaller amounts of energy, so the photons have a lower frequency. The main jump down releases these lower frequency green photons, so that's what we see. In other words, the electron arrangements in the Green Fluorescent Protein are such as to absorb invisible ultraviolet photons and mainly release visible green photons.
The other clever thing is that the Green Fluorescent Protein gene is bundled with genes that give the bacteria resistance to the antibiotic called ampicillin. There is also a gene that means that the Green Fluorescent Protein gene only works in the presence of a sugar called arabinose. So the photograph at the top is for a plate treated with ampicillin and arabinose.
