There’s a small dark hut in the Museum Court which has an astonishing surprise inside: fluorescing minerals, glowing in beautiful and vivid colours.
Minerals are the natural chemicals that make up the Earth’s rocks, but just a few have a chemical activator that makes them fluorescent under ultraviolet light. This chemical may be an essential part of the mineral’s make-up, but sometimes it is just an impurity.
Although invisible to humans, ultraviolet light is part of the electromagnetic spectrum and has a bit more energy than visible light. When it shines on a fluorescent mineral energy is absorbed by the electrons in the mineral’s atoms, bouncing them into more energetic states. But as they return to their previous states some energy is emitted as visible light – the coloured light that we can see.
Inside the mineral hut there are two kinds of ultraviolet light: longwave and shortwave, which can produce different responses. Minerals from the world-famous Franklin Furnace in New Jersey, USA, for example, will fluoresce in scarlet, green and yellow under shortwave light, but barely glow under the longwave lamp.
Fluorescence is not confined to the mineral kingdom. Finger nails, bones, teeth and egg shells can all be fluorescent too. The word ‘fluorescent’ itself comes from the name of another common mineral, fluorite (calcium fluoride), which can fluoresce so strongly that it emits an eerie violet purple glow when bathed in bright sunshine outdoors.