LIGHT 



3423 



LIGHT 



Length of Light Waves. Heat waves as well 

 as light waves are transmitted from a glowing 

 body, and the only difference between these is 

 their length, the average heat wave being longer 

 than the average light wave. The longest light 

 waves are those that produce red light and the 

 shortest those that produce violet light. The red 

 waves and violet waves are therefore the two 

 extremes of the range of vision of the normal 

 human eye. Waves that are longer than those 

 producing red light effect the skin and cause 

 the sensation of heat; those shorter than vio- 

 let rays bring about certain chemical changes. 

 They affect the sensitive plate of the camera 

 and make possible the photographing of ob- 

 jects not visible to the eye; they fade artificial 

 coloring matters, assist in plant growth, destroy 

 disease germs and cause sunburn and tan. 



The vibrating molecules that start ether 

 waves have a frequency of vibration beyond 

 the power of the mind to grasp. The shorter 

 waves have a greater number of vibrations per 

 second than the longer, though all travel at 

 the same rate of speed. Blue waves, for in- 

 stance, which are much shorter than red waves, 

 have a vibration frequency of 600 trillions a 

 second. 



Speed of Light. If we could travel to the 

 sun, which is about 93,000,000 miles away, in 

 an airship having the same velocity as light, 

 we would make the trip in eight minutes and 

 nineteen seconds. Light travels through space 

 at an amazing speed, about 186,000 miles a 

 second. One of the fastest of America's trains 

 runs from Chicago to New York in twenty 

 hours; light can cover the distance, 960 miles, 

 in one two-hundredth of a second. It can 

 readily be seen, therefore, that for all distances 

 on the earth this rate is instantaneous. When 

 we consider that the most distant stars that we 

 can see in the firmament at night are so far 

 away it takes their light about 5,000 years to 

 reach us, we can form some idea of the infinite 

 vastness of the universe. Light travels a little 

 more slowly in air than in a vacuum, and still 

 more slowly through such denser substances as 

 glass and water. 



A Straight-Line Advance. Did you ever 

 stop to think why you cannot see around a 

 corner, or why, when looking at an object 

 through a tube, the view of it is cut off if the 

 tube is bent? The reason is that licht in 

 passing through a medium of uniform density 

 advances in straight lines. The surveyor knows 

 that he can measure angles exactly because 

 the light which comes from a distant object 



to his instrument follows a straight path. Upon 

 this fact also depends the forming of a shadow 

 when a body obstructs the passage of light. 

 If a screen through which light cannot pass is 

 placed between a lamp and a wall, for instance, 

 that part of the wall opposite the screen will 

 be in the shadow. The light waves, traveling 

 in a straight line, strike the obstructing body; 

 some of the waves are reflected back to the 

 eye and the rest are absorbed, but none can 

 pass out of their course and around the screen. 

 Hence the unilluminated space behind it. 



Transmission through Substances. An ob- 

 ject like that described in the preceding para- 

 graph, which cuts off light waves from a lumi- 

 nous body by absorbing them, is called opaque ; 

 light cannot pass through it. Some substances, 

 such as glass, transmit it so easily that we can 

 see clearly objects through them. We call such 

 bodies transparent. Other substances, such as 

 oiled paper, transmit light imperfectly, and ob- 

 jects are seen through them dimly. They are 

 known as translucent. There is no sharp divid- 

 ing line in the use of these terms, for the same 

 substance under different conditions may be 

 transparent and opaque, opaque and translu- 

 cent, etc. Goldleaf, for example, though ordi- 

 narily opaque, can be made so thin that one 

 can see through it ; the water in a clean, shallow 

 pond is transparent, but that in a deep lake is 

 often opaque. Familiar examples of this fact 

 can be multiplied. 



Intensity of Illumination. A person makes 

 a practical application of one of the laws of 

 light every time he moves a chair nearer to 

 the window or to a lamp so a better light may 

 fall on the book he is reading. This law may 

 be stated as follows: The intensity of light 

 decreases as the square of the distance in- 

 creases. Stated another way, an electric globe 

 two feet from an object will throw four times 

 as much light upon the object as when it is 

 four feet away, and sixteen times as much as 

 when it is eight 

 feet away. 



Reflection of 

 Light. We have 

 already learned 

 that bodies be- 

 come visible by 

 the light which 

 they send back, 

 or reflect, to the 

 eye. Reflection is defined as the turning back 

 of light waves, by the substance upon which 

 they strike, into the medium through which' 



