Reproduced from " The Forces of Xature" 



A SOAP BUBBLE 



The iridescent colours sometimes seen on a soap bubble, as in the illustration, may also be seen in very fine sections of crystal*, 

 in glass blown into extremely fine bulbs, on the wings of dragon-flies and the surface of oily water. The different colours correspond to 

 different thicknesses of the surface. Part of the light which strikes these thin coatings is reflected from the upper surface, but another 

 part of the light penetrates the transparent coating and is reflected from the lower surface. It is the mixture of these two reflected 

 rays, their "interference" as it is called, which produces the colours observed. The "black spots" on a soap bubble are the places 

 where the soapy film is thinnest. At the black spots the thickness of the bubble is about the three-millionth part of an inch. If 

 the whole bubble were as thin as this it would be completely invisible. 



