292 OUR PHYSICAL WORLD 



Suppose, for instance, points a and b (Fig. 13 yZ)) represent the 

 opposite ends of an arrow seen reflected in a concave mirror, 

 these points being slightly nearer the mirror than its focus. 

 Follow two rays of light, the outer or the marginal rays of a 

 pencil of light, from point a to the mirror. When these are 

 reflected into the pupil they are less divergent than when they 

 left a. They will seem, therefore, to come from a point back of 

 the mirror and farther from the mirror than a is in front of it. 

 Similarly, b will appear at b', and the arrow will seem larger than 

 it is. So a dentist uses a small concave spherical mirror to see 

 his work on a tooth, and thereby magnifies the cavity he is 

 cleaning and filling. 



An image is formed by a lens because the light entering and 

 leaving it is bent from its straight course or is refracted. Such 

 refraction always occurs when rays of light go into or out of 

 an optically more or less dense medium than the one in which 

 they were traveling, and the refraction occurs at the line of 

 demarcation of the two media. Thus light entering water from 

 air is refracted as it enters the water. Optical density and 

 ordinary physical density must not be confused. Thus carbon 

 disulphide is a liquid and not physically as dense as glass; yet 

 optically it is more dense than most glass, that is, it bends the 

 ray of light entering it more than does glass, or to put it in another 

 way, it has a higher refractive index than glass. 



One may perform a simple experiment that will help clarify 

 this notion of refraction. Put a penny in a bowl that sits on 

 the table. Stand where you can just see the penny over the 

 edge of the bowl, and then step back until you just cannot see 

 it. Have some other person pour water into the bowl carefully 

 so as not to move the penny. The far side of the penny begins 

 to appear, and as the level of the water rises you see more and 

 more of it until it is all in sight. Evidently the rays of light, 

 coming from the penny over the edge of the bowl, go above your 

 eyes before the water is added, and after that are bent down 

 so that they enter your eyes. (See Fig. 139.) If one draws a 



