Incidentally...

Many things glow (or fluoresce)with visible light when ultra-violet light shines on them. Washing powders contain blue fluoresces so that items will look bluish-white whether they are clean or not. Nearly all white paper is similarly treated. Banknotes should not fluoresce in u-v so this is an obvious test for forgeries. Ordinary fluorescent tube lights are basically u-v lamps. The inside of the tube is coated with material that will fluoresce with white light when u-v from the mercury vapour in the tube shines on it. The vapour is not hot, but is made to glow by electricity.

All hot things give out radiation. If very hot, like the sun (6000°C), they emit a lot of the dangerous ultra-violet light as well as all the visible and infra-red spectrum. Ultra violet light damages body cells. Our skin is stimulated into producing its own blocking substance, melanin, by a sensible dose of u-v. This is called acquiring a tan!

If the dose is excessive, the skin is damaged and becomes inflamed and painful. This is what usually happens first! Sun cream, which contains oils opaque to u-v, should be used to limit the dose until melanin takes over.

 
   
Did you know?

Many things glow (or fluoresce) with visible light when ultraviolet light shines on them. Washing powders contain blue fluorescers so that things will look bluish-white whether they are clean or not. Nearly all white paper is similarly treated. Banknotes should not fluoresce in ultraviolet so this is an obvious test for forgeries.

Ordinary fluorescent light tubes are basically ultraviolet lamps. The inside of the tube is coated with material that will fluoresce with white light when ultraviolet from the mercury vapour in the tube shines on it.

All hot things give out radiation. If very hot, like the sun (6000°C) or a welder's electric arc, they emit a lot of the dangerous ultraviolet light as well as all the visible and infra-red spectrum.

Ultraviolet light damages body cells. Our skin is stimulated into producing its own blocking substance, melanin, by a sensible dose of ultraviolet. This is called acquiring a tan!

If the dose is excessive the skin is damaged and becomes inflamed and painful. This is what usually happens first! Sun cream, which contains oils which absorb ultraviolet, should be used to limit the dose until melanin takes over.

Different grades of sun cream contain oils to absorb different amounts of ultraviolet light. If the sun cream's "factor" is 15, for example, it only lets through one fifteenth of the ultraviolet light falling on it, so you can stay out fifteen times longer! "Total block" cream is designed to cut out the ultraviolet completely.

The earth has a natural "sunscreen" - the ozone layer, high up in the atmosphere. Ordinary oxygen (O2) converts into ozone (O3) when it absorbs ultraviolet light. Ozone is a good ultraviolet light absorber, but soon converts back to ordinary oxygen. Ultraviolet light converts this back to ozone, so that the ultraviolet absorption can go on and on, unless nasty CFC gases foul up the process.

Sun creams contain a variety of ultraviolet-absorbing compounds. Many are derived from flavenoid (oxygen ring) compounds which occur in the outer cells of plants. Some of these compounds provide ultraviolet protection, others absorb particular colours of visible light strongly, so act as pigments in petals. Some flavenoids are even anti-bacterial!

The "factor" number of a sun cream tells you how many times longer you can stay out in the sun to get the same ultraviolet dose as when unprotected.

Ultraviolet light does other things, besides promoting unwanted chemical reactions. It makes lots of materials fluoresce, that is, glow with ordinary light. Washing powders contain fluorescent materials which glow blue in the ultraviolet part of sunlight, to help clothes to look whiter than white! Wonder plastic, used to make the figures in the exhibit, is impregnated with material that glows red.

The insides of fluorescent light tubes are coated with materials that glow brightly when they absorb the ultraviolet light produced by the mercury vapour glowing inside the tube. A mercury vapour tube, without the fluorescent lining, produces the ultraviolet light in the exhibit which makes the Wonder plastic glow. The backs of the figures are covered with artificial skin (as used in a hospital burns unit) and sun cream.

 
 
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