310 PHYSIOLOGICAL PHYSICS. ichap. xxiv. 



bent upwards, owing to the diminished density of the 

 air by contact with the heated ground, so as to reach 

 the eye of .an observer at a. The observer refers the 

 object from which the rays proceed 

 to the direction in which the rays 

 reach him, and thus an inverted 



ff ' t 



J&-" image of A would be seen at A', and, 



Fig. 136. Eefrac- in the same way, an inverted image 

 of a landscape would be seen. 



In the case of a ray of light passing from one 

 medium into another less dense, the angle of refrac- 

 tion must not be greater than a right angle, else the 

 refracted ray will not emerge from the dense medium, 

 but will be reflected at its surface. If the angle of 

 refraction be a right angle, the ray is refracted 

 parallel to the surface of the dense medium. The 

 value of the angle of incidence giving rise to the right 

 angle of refraction, is called the critical angle, because 

 any greater angle will prevent the emergence of the 

 ray. When, owing to the greatness of the angle of 

 refraction, the ray does not emerge, the occurrence is 

 called TOTAL REFLECTION. 



The laws of refraction are that the incident 

 and refracted rays are in the same plane, and that 

 there is a definite relationship between the angle of 

 incidence and the angle of refraction. The angle of 

 incidence is that made by the incident ray and the 

 normal ; that of refraction is made by the refracted 

 ray and the normal. In Fig. 135, NN' is the normal, 

 the angle i is the angle of incidence, and the angle r 

 is the angle of refraction. The relation between 

 these two angles is such that their sines are in a 

 constant ratio. This is .expressed by saying that 



O i y Q n 



= a constant quantity designated by /t. This 

 sine r J J 



constant ratio is called the INDEX OF BEFRACTIOH, 

 From air to water the index is four- thirds, or 1 '33. 



