Light 181 



Light waves are very short. Even the longest are only about 

 0.000027 of an inch in length. The most careful measurements 

 show that they travel at the rate of about 186,000 miles, or seven 

 and one-half times around the earth, in one second. 



As we watch the ocean waves come up to the shore, we notice 

 that they travel one behind the other at regular intervals. If we 

 were to count the number which come up per minute we would 

 have a means of measuring the rate at which they are formed. 

 This is sometimes called the frequency of the wave motion. The 

 frequency of the longest wave motion which we can recognize 

 as light is about 400 trillions per second, while the shortest 

 wave motion which results in light is about 700 trillions per 

 second. 



There are many waves in ether which are longer than the 

 light waves. Those which are only slightly longer are known 

 as heat waves. These heat waves explain how the heat comes 

 to us from the sun. They travel at about the same rate as light. 

 Radio waves are of a similar nature, only much longer, some being 

 miles in length. 



A light ray is a single line of light, too small to be visible. 

 A group of parallel rays is called a beam of light. If the rays 

 are not parallel, but come to a point, they form a pencil of 

 light. 



Some materials stop light rays and others permit them to pass 

 through. A material through which light passes so perfectly 

 that we can see objects on the far side of it is called transparent. 

 Window glass is an example of transparent material. A material 

 through which light passes, but not perfectly enough so that 

 objects can -be seen on the other side of it, is called trans- 

 lucent. A piece of thin paper and a pane of frosted glass are 

 translucent. If no light passes through a material it is said to be 

 opaque. 



Color. The different colors which we see are due to the 

 variations in frequency of the light waves. The longest waves, 

 which have a frequency of about 400 trillions per second, appear 

 to us as red, while the shortest, which have a frequency of about 

 700 trillions, appear as violet. Other waves which range between 



