Every person has unquestionably observed that 

 when a spoon is placed in a tumbler of water it is 

 apparently bent at the surface of the water, or when 

 looking at an object lying in the bottom of a dish, it is 

 apparently at a different point than when viewed from 

 the side. This is caused by the deflection or bending 

 of the rays of light as they pass from one transparent 

 medium into another of greater or less density and is 

 called refraction. The amount of refraction increases as 

 the difference in the density of the two media becomes 

 greater. We also know that in viewing objects through 

 a glass prism, they apparently lie in a direction differ- 

 ent from their real one. The amount of this deviation 

 depends for the one factor upon the density of the 



glass composing the 

 prism and for the 

 other upon its shape. 



Fig. i represents 

 a cross section of 

 a prism and shows 



how the ray on 



- , r 



entering at the first 



surface undergoes refraction and how on emerging at 

 the second surface a second refraction takes place. 

 It will be noticed that the light is bent downward 

 or toward the base of the prism and if the prism 

 be imagined reversed with its base upward, the action 



