364 THE SUN'S GIFT OF LIGHT 



SUMMARY 



The sun is the source not only of almost all the heat of the 

 earth but also of practically all its light. Light is just as 

 essential to life as heat is. No comprehensible figures will 

 express the intensity of the sun's light, using the candle 

 power as a measure. The intensity of light varies inversely 

 as the square of distance from its source. 



All objects except self-luminous bodies are seen by re- 

 flected light. Objects on the earth reflect both heat and 

 light. The angle at which a ray of light is reflected is equal 

 to the angle at which the ray strikes the reflecting surface. 



Light travels at the rate of about 186,000 miles per second. 

 When it travels through a uniform medium, it goes in a 

 straight line; but when it travels through media of 

 varying densities the rays are bent or refracted. The 

 bending of light rays in passing from one medium to 

 another is turned to great advantage in the use of lenses 

 which may be so constructed as to bend rays of light in any 

 desired direction. 



When a ray of white light is passed through a prism, it is 

 not only refracted but is also separated into different colors. 

 Light is a form of wave-motion, and the infinitesimally 

 small wave-lengths of the various colors differ from one 

 another. This accounts for the different degrees of bending 

 of the various color rays when passed through a prism. The 

 band of colors into which white light is separated is called 

 the spectrum. The color of a non-luminous substance 

 depends on the kind of light it transmits or reflects. When 

 a substance is brought to a white-hot vapor, it has a char- 

 acteristic spectrum. By combining the prism with the 

 "telescope, scientists have an instrument called a spectroscope, 



