Rainbows hold the secret of colour

Press, 6 November 1984, Page 11

Rainbows hold the secret of colour

The strong : white light which comes from the sun is made up of all the colours of the rainbow. Sir Isaac Newton discovered this in 1666. He passed a beam of sunlight through a triangular block of glass, or prism, and obtained the colours of the rainbow in a band on a screen in front of the prism. Using a second prism he was able to combine the colours, which are called the spectrum, to form white light again. Another way to demonstrate the fact that white consists of all the colours, is to colour a disc with the colours of the rainbow and spin it quickly. The disc will appear whitish when it is spinning. Newton’s experiment with the prism led scientists to a better understanding of colour, one of nature’s mysteries.

They discovered that objects are seen as a certain colour because they soak up all the colours of the light shining on them except one. That one colour is reflected and is the colour seen by the viewer. In this way a red rose soaks up all the colours but red which it throws back for people to see. A green leaf throws back green. The dust hanging in the sky reflects only blue, so colouring the

sky. White surfaces are those which take in no light, but reflect it all. Surfaces which take in all the colours and send none back to the eyes are called black. In coloured light, an object will not appear in its natural colour unless the light is the same colour as the object. A red tie would look red in a red light but black in a blue light. In lights of other colours it would look different again. The skin of a person standing in a yellow street light does not. look natural. This is because the light does not contain all the colours of the spectrum, and objects can reflect only col-

ours which exist in the light shining on them. Artificial white light does not always produce the same effect as sunlight. Sometimes material in a lighted shop can look different when taken out into the sunlight. The way the eye receives colour is a complicated process which involves chemical changes at the back of the eyeball. People who cannot see

colours are said to be colour blind. Often a person with this condition cannot recognise one particular colour only. A paint box contains squares which each absorb all the colours except one, and that one is" the colour each square is known by. Useful paints are red, yellow and blue; by mixing these in the right proportions it is possible to make almost any colour.