CH. xxxm. COLOURS CAUSED BY INTERFERENCE. 321 



when the sunlight falls on the thin film of the bubble, a 

 little of the light is reflected straight back to the eye from 

 the surface a, Fig. 5 9 ; but most of it passes on to the 

 second surface b, and there again some is reflected, so that 

 two sets of waves are constantly reaching the eye, one from 

 a and one from b. These two sets meet before they come 

 to our eye, and we have just seen that it depends entirely 

 how they meet what colour we see. 



Suppose the film is just thick enough for the two rays 



FIG. 59. 

 Reflection of Light from the two surfaces of a Soap-bubble.* 



R, Ray of light, part of which is reflected from the surface a, and part from the 

 inner surface, b, to the eye. 



to meet when the red waves of each are rising ; we shall 

 then have a full red wave upon our eye. But in that case, 

 as the violet waves are a different length, they will not have 

 met as friends, but as foes, one up and the other down, and 

 will destroy each other ; and so will the waves of all the 

 other colours, because they are not of the same length as 

 the red waves. Therefore the only impression on our eye 



1 The film of a soap-bubble is really only the thickness of a fine 

 line even in the thickest part ; but it was necessary to exaggerate the 

 two surfaces in the diagram to show the passage of the ray of light. 



