CH. xxxii. COLOURS CAUSED BY INTERFERENCE. 307 



If this experiment is made with light of one colour only, 

 as, for example, with light which has been passed through 

 red glass, and so is composed only of red rays, then the 

 bands are simply dark and light. But if sunlight is used 

 another curious effect is seen, namely, the bands are faintly 

 tinged with the colours of the rainbow; and this too Young 

 showed to be beautifully explained by the Undulatory Theory. 

 It was stated at p. 177 that the colour of the light which 

 reaches our eye depends upon the rapidity of the vibrations 

 of the ether, just as the sound of a note upon our ear 

 depends upon the rapidity of the vibrations of the air. 

 Consequently the waves of the prismatic colours are of 

 different lengths, so that when the two rays of light meet 

 behind the card the waves of the various colours do not 

 all arrive together. For example, those waves which cause 

 us to see the colour violet are much shorter and more 

 rapid than those which cause us to see red. Therefore, 

 when the red waves meet each other as friends (as in 

 Fig. 48), and make a strong vibration, the violet ones will 

 meet each other as foes (as in Fig. 49), and interfere with 

 each other ; and so we shall see a bright red stripe made by 

 the strong red wave, while the violet waves will be destroyed 

 A little farther on the violet waves will meet as friends, and 

 then we see a violet streak, while the red ones will in their 

 turn be destroyed. 



Colours on the Soap-bubble. The beautiful colours of 

 the soap-bubble are caused in this way, and an explanation 

 of them will help you to picture to yourself this effect of the 

 interference of light. If you have ever blown a well-shaped 

 soap-bubble, and watched it settle down quietly where there 

 is no wind to disturb it, you cannot fail to have noticed the 

 colours which appear upon it. If the bubble is very perfect 



