LIGHT 227 



it will seem to be bent where it enters the water. Place 

 a coin in a cup with opaque sides so that it is just out of 

 view (Figure 200) . Keep the eye fixed on the same place 

 and fill the cup with water. The coin will now be visible. 

 Figure 201 represents the bending of a beam of light as 

 it enters and leaves the water. The reason that it is bent 

 is that light travels slower in water than in air. The 

 lower side of the beam enters 

 the water first and is retarded 

 so that the upper part of the 

 beam gets ahead,- thus bend- ^%/A Air 



ing it toward a perpendicular 

 as shown in the figure. As 

 the beam leaves the water, 

 the lower part reaches the 

 air first and immediately in- 

 creases its speed, while the FIG. 201. AS light passes from 



/ , . *" e alr m t the water at an angle 



Upper part Of the beam IS other than a perpendicular it is bent 



still in the water. Thus it is or refracted - 

 bent again but in the opposite direction. If the beam 

 strikes the surface of the water at right angles, it will not 

 be bent in either direction. 



Light is bent or refracted when it passes obliquely from 

 one transparent medium to another of different density. 

 This principle is of great importance, since it is the one 

 involved in the eye and in nearly all of our important opti- 

 cal instruments, including the microscope, the telescope, 

 camera, and stereopticon. 



Lenses. There are a great many practical uses made 

 of lenses. They are used in cameras, microscopes, tele- 

 scopes, spectacles, field and opera glasses, stereopticons, 

 and other similar instruments. 



A lens is usually made of glass and has two curved sur- 



