Andrew Wallace, of Nayland College, asks :-

Why is ultraviolet light so damaging?

John Campbell, a physicist at the University of Canterbury, responded.

Because ultraviolet (uv) light has sufficient energy to break most chemical bonds.

In general, atoms join together to form molecules. They are bonded together by attractive forces which are of an electrical nature. We can break up a molecule into smaller molecules, or even into its component atoms, by giving the atoms sufficient energy so that they fly apart thereby overcoming their attractive bonds.

One way is to use light. The smallest packet of light is called a photon and the energy of the photon depends on the colour of the light. Blue light has about twice as much energy per photon as red light does.

A photon of visible light has enough energy to break some weaker chemical bonds. Sometimes this is useful, for example, in photosynthesis in plants or the photographic process. Sometimes this is just a nuisance, such as when some fabrics fade after long exposure to bright sunlight as the dye molecules are progressively broken to become non-coloured molecules.

A photon of ultraviolet light has about ten times more energy than a photon of visible light so can break almost any chemical bond. Hence it is dangerous. The chemicals in our skin break down when sunbathing (thus ageing the skin and sometimes initiating a cancerous growth). If it were not for the ozone layer in the Earth's upper atmosphere we would be in big trouble. Most of the more dangerous ultraviolet light emitted by the sun is absorbed in breaking up ozone molecules.