798 



THE SENSE OF SIGHT 



object. A real image, therefore, is produced by the reflected rays themselves, and 

 may be observed with the aid of a screen properly adjusted at their points of inter- 

 section. Divergent rays, on the other hand, are supposed to be projected directly 

 through the mirror and are seen as if they proceeded from its other side. In 

 the latter case, the image has no real existence, but is effected by the prolon- 

 gations of the reflected rays backward. This is called a virtual image. Obviously, 

 therefore, a real focus is formed by the reflected rays themselves, while a virtual 

 focus is formed by their prolongations backward through the mirror. 



Refraction. If a ray of light is made to pass from one medium 

 into another in a perpendicular direction, it is not deviated from its 

 course. The contrary result, however, is obtained if it is made to 

 enter in an oblique direction. To this phenomenon the term of 

 refraction has been applied. It is to be remembered that not all the 

 rays of a certain beam of light are refracted, because some of them are 

 reflected from the surface in accordance with the character of the 

 medium into which they have been directed. Those that actually 

 enter the denser medium are refracted, because their velocity of pro- 

 pagation is now less than it was in the rarer medium. The degree 

 of refraction differs with the relative densities of the two media. 

 Supposing that we are dealing with air and water separated by a thin 



FIG. 407. DIAGRAM ILLUSTRATING FIG. 408. DIAGRAM ILLUSTRATING REFRACTION. 

 REFRACTION. 



layer of glass (Fig. 407), it will be found that any ray directed verti- 

 cally to the surface of the latter (AB), is not deviated from its course 

 (BC). Any incident ray, however, which strikes the surface of the 

 water obliquely (DB\ is deflected (BE) toward the perpendicular 

 AC. The angle of incidence ABD^ is then larger than the angle of 

 refraction CBE, and naturally, this angle becomes the smaller, the 

 greater the refracting power or density of the second medium. The 

 ratio between the angle of incidence and the angle of refraction con- 

 stitutes the index of refraction.' When passing in the opposite direc- 

 tion (Fig. 408), namely from a medium of greater into one of lesser 

 density or refractive power, the ray BE is bent away from the perpen- 

 dicular rendering the angle of refraction greater than the angle of 

 incidence. Thus, taking the index from air to water to be ^ and from 

 air to glass %> the course of the ray in the opposite direction would 

 show an index of % and % respectively. 



The first law of refraction states that the refracted ray is in the 



