REFRACTION 123 



A single convex lens used to magnify small bodies is 

 called a simple microscope. A compound microscope is a 

 group of several convex lenses in a tube, so placed that 

 each magnifies the image formed by the one before it. 

 With such an instrument objects may seem to be hun- 

 dreds of times their real size. 



144. The Telescope. A telescope is a device for view- 

 ing distant bodies. It generally contains two convex 

 lenses, an objective o (Fig. 97), and an eyepiece e. The 

 objective serves to collect as many rays as possible, and 



FIG. 97 



the eyepiece magnifies the image formed by the objec- 

 tive. A few telescopes are enormous. One in the 

 Yerkes Observatory, in Wisconsin, has an objective 

 forty inches in diameter. Some of these can magnify 

 images over five thousand times. 



o 



QUESTIONS 



1. What is meant by refraction ? State any examples of refrac- 

 tion that are familiar to you. 



2. Name the conditions that are necessary in order that a wave 

 may be refracted. At what point does refraction occur ? 



3. Explain the cause of refraction. Show why a wave is not 

 refracted when it meets a surface at right angles. 



4. What is a lens? How \s a lens related to a prism in its 

 effect upon light rays? Name the two general shapes of lenses. 



5. What, in general, is the effect of a concave lens upon 

 parallel rays? of a convex lens? 



