Stick a red or orange LED light into liquid nitrogen and the color will change. Why would cooling down an electrical device give it a higher-energy light? Find out!
Above you can see footage of an LED light being dipped in liquid nitrogen. The light starts out relatively low-energy, but once it cools in a vat of liquid nitrogen it turns from an orange, to a yellow, and finally to a yellow-green. This is an increase in the frequency of the light, and an increase in its energy. But cooling an object sucks energy away. How does a cooling process result in higher-energy photons being emitted?
Let's look at how photons are emitted in atoms. When an atom is at rest, its electrons sit at their lowest possible energy-levels. Shoot a photon in, and one of the electrons hops to a higher level. When it descends again, towards the proton, the electron gives off a photon. The bigger the hop between levels, the higher-energy the accompanying photon.
And LED light works by running its electrons along a negatively charged band, and then having them fall through little holes towards a positively charged band. As they fall (or hop, depending on your perspective) they give off photons. The difference in energy level between the positive band and the negative band is called the bandgap. The bigger the bandgap, the bigger the fall, and the higher-energy the photons. The electrons, starting out, have a little thermal energy in them. When that thermal energy is sucked out, for example by a vat of liquid nitrogen, the difference of the energy levels on the negatively charged band and the positively charged band is increased. The bandgap increases, the fall increases, and the photons emitted get more energetic. As the LED cools, it emits higher-energy, higher-frequency photons, and we see it change from orange to yellow to green.
[Via SMU Astronomy and Physics.]