DEVICES FOR SEEING 



2 93 



line perpendicular to the surface of the water at the point at 



which the ray of light leaves the water and enters the air, point b, 



Figure 140, one may state the direction of the refraction as 



away from the perpendicular when the 



ray passes from an optically dense 



medium to a less dense one (water to 



air), and toward the perpendicular 



when the light moves in the opposite 



direction, as would be the case if eye 



and coin interchanged positions in this 



experiment. In spearing a fish, from 



behind it, one must aim the spear at 



its tail in order to hit its body, or 



FIG. 139. Diagram show- 

 ing refraction of light from an 

 object in water. 



if it is lying in deep water the spear must be thrust at a point 



back of the fish in order 

 to hit it at all. 



The amount of the 

 refraction depends on 



b the relative density of 



the two media. Air is 

 taken as the standard, 

 and when we say that 

 a given sort of glass 

 has an optical density 

 of 1.5, we mean that it 

 is half again as dense 

 as air, or that light 

 travels through it only 

 two- thirds as rapidly as 

 through air. Practi- 

 cally we apply this in 

 tracing the course of the ray as follows : Suppose ab (Fig. 140) is 

 a ray of light which at b enters the plane surface of a piece of 

 glass with a refractive index of 1.5. With point b as a center 

 and a radius of i (in the diagram the radius is i inch), strike 



FIG. 140. Diagram showing method of finding 

 the path of a ray of light entering glass. 



