SUMMARY 67 



unit for measuring the amount of heat is called the 

 calorie. 



Light moves at the almost unbelievable rate of nearly 

 two hundred thousand miles a second. It goes straight 

 except when passing at an angle through media of different 

 densities. It is then refracted. Its intensity varies in- 

 versely as the square of the distance from the source. 

 When light is reflected from any smooth surface like a 

 mirror, the angle of reflection is equal to the angle of 

 incidence. 



Sound, like light, is a wave motion. But sound waves 

 can travel through substances that shut out all light, and on 

 the other hand light waves can travel through a vacuum 

 that shuts out all sound. So the intensity of light or 

 sound depends largely upon the medium through which it 

 is conveyed. 



QUESTIONS 



Why would it be true to say that all artificial light is the sun's 

 light? 



Germany uses the Centigrade thermometer scale. If the tempera- 

 ture of Berlin is reported as 20 C. what would the corresponding 

 temperature be in the thermometer scale generally used in the 

 United States? 



Why are iron and type metal better suited for casting than copper 

 and zinc ? 



In what three ways is heat transferred? 



Describe how you could prepare from the ordinary materials you 

 have at hand a crude, inexpensive fireless cooker. 



Ponds near the Great Lakes freeze entirely over. Why do not the 

 Great Lakes freeze? 



What experiences have you ever had illustrating refraction ? 



If a boy is reading two feet from a light and moves to a distance of 

 eight feet, how much ought the light to be increased to enable him to 

 read with the same ease? 



When the sun is shining brightly, why is it hotter standing on a 

 smooth pavement than on the grass? 



How long does it take light to come from the sun to the earth ? 



