352 THE SUN'S GIFT OF LIGHT 



to reflect the heat of the sun on to boilers to run steam 

 engines. 



The smooth surfaces of houses often reflect so much of 

 the light falling upon them that the glare is thrown into 

 the windows of surrounding houses into which the sun 

 itself cannot shine. If one stands in the right position, the 

 reflection of trees and other objects can be seen in a smooth 

 lake. But the reflection cannot be seen if the position of 

 the spectator is much changed. The reflected ray must 

 therefore maintain a certain relation to the ray that strikes 

 the surface from the object. 



In Experiment 102, when the pencil was held perpen- 

 dicular to the mirror at the point where the rays touched the 



mirror, it was seen that both 

 the ray from the window and 

 the reflected ray made about the 

 same angle with it. These two 

 FIGURE 100 angles are respectively called the 



angle of incidence and the angle 



of reflection. By most careful experimentation it has been 

 found that the angles between each of these two rays, and 

 the line drawn perpendicularly to the reflecting surface 

 are always equal, or in other words the angle of reflection 

 is always equal to the angle of incidence. (Figure 100.) 

 This explains why, if you are standing in a room at 

 one side of a mirror, you can see in the mirror only the 

 opposite side of the room. We are accustomed to a 

 similar law of reflection when we bounce a ball on the floor 

 for some one on the opposite side of the room to catch. 



The Speed of Light. In the latter part of the seven- 

 teenth century a Danish astronomer by the name of Roemer, 



