THE ACTION OF LIGHT 



a ray of light through a prism is shown in the 

 diagram. As the ray passes from air into glass it 

 is bent, because glass is more dense than air; it 

 is bent once more on leaving the prism because air 

 is less dense than glass. 



Now lenses are made of various shapes, and those 

 with two outwardly curved surfaces are known as 

 double convex lenses. A double convex lens is 

 usually made with both its surfaces equally curved 

 and in the finer optical work great care is taken to 

 ensure that this is the case. For certain purposes, 



however, as we shall learn in a moment, one or 

 other of the faces only may be much more curved 

 than its companion and this may be carried to such 

 an extreme that one face is fiat, the lens is then 

 known as plano-convex. Lenses may also have 

 inwardly curved faces, if both are of this design 

 they are called double concave; if one face is flat 

 and the other inwardly curved they are known as 



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