When such a thing as a curtain or a piece of cloth fades, it is because the chemical substances in it that give it its colour have been partly destroyed. Most of this Colouring matter Consists of substances which can be burnt, or oxidized, and if a thing is exposed to the air, of course there is plenty of oxygen round about it.
The Sun's rays destroy the colour, because they help on this chemical change that we call oxidation. Every photograph is taken because of the power of the Sun's rays to produce chemical changes, and the fading of a piece of cloth exposed to the Sun is really very much the same as what happens in a photographic plate. The part of the Sun's rays which has this chemical power is the part which produces the colours of blue and violet when it strikes our eyes, and also two or three colour-notes, as we might call them, higher up than the violet, which our eyes cannot see, and which are called ultraviolet.
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