LIGHT AND COLOUR 



sider the usual colours of near objects seen in 

 white daylight, and ask why a rose is red, and 

 why the grass is green. How does the rose 

 change the white light of the sun into red ? 

 We cannot find an answer to this question till 

 we know more about the nature of white light. 

 Is a beam of white light all one kind of thing, 

 or does it contain different kinds of light, some 

 of which may be treated in different ways from 

 others by the bodies on which they fall ? An 

 experiment we may all of us make throws light 

 on this problem. Glass is a colourless sub- 

 stance, which usually lets white light through it 

 unchanged. Yet, if we take a piece of glass 

 with a sharp wedge-like edge, and hold it up to 

 the light, bright colours are seen. Water too 

 shows the same effect. In large masses it is 

 colourless, or nearly so, and a glass full of water 

 lets the light of the window pass through it and 

 still looks white. When divided into fine spray 

 or rain, however, the same colours that we found 

 with the glass wedge may sometimes be seen. 

 When water falls in spray over rocks, or when 

 a shower has passed over us, and the sun is 

 shining from behind us, some of the light pass- 

 ing through the drops will be bent back to our 

 eyes and form a rainbow. Now all these things 

 are easily explained if we imagine that what our 

 eyes see as white light is really not simple but 

 complex, made up in a certain balanced propor- 

 tion of coloured lights, which, when perceived 



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