REFLECTION OF HEAT AND LIGHT 



351 



the amount of light on the book four times ; if one third 

 as great, nine times. (Figure 99.) 



Reflection of Heat and Light. Experiment 102. In a dark- 

 ened room reflect by means of a mirror a beam of light from a small 

 hole in the curtain, or from some artificial source of light, on to 

 a plane mirror lying flat upon a table. If there is not sufficient 

 dust in the air to make the paths of the rays apparent, strike two 

 blackboard erasers together near the mirror. Hold a pencil ver- 

 tical to the mirror at the point where the rays strike it. Compare 

 with each other the angle formed by each ray with the pencil. 

 Raise the edge of the mirror, and notice the effect on the reflected 

 ray. Place the pencil 

 at right angles to this 

 new position of the 

 mirror, and compare 

 the angles in each case. 

 How do the sizes of the 

 angles on either side of 

 the pencil compare ? 



It has already been 

 stated that the moon 

 shines by reflected 

 light. It is a matter 

 of common observa- 

 tion that objects on 

 the earth reflect both 

 heat and light. In 

 the summer, the 

 walls of the houses 

 and the pavements 



of the streets sometimes reflect the heat to such an extent 

 that it becomes almost unbearable. In countries where 

 the sun shines brightly nearly all of the time, as in the 

 Desert of Sahara, reflectors have been so arranged as 



A REFLECTION ENGINE 



This engine uses the rays of the sun instead 

 of coal in heating its boiler. 



