104 



HOW WE LIGHT OUR HOMES 



Darken a room and allow a little sunlight to enter through 

 a small opening. Hold a prism in the sunlight so that the 

 colors produced are thrown on one of the walls of the room. 

 Hold a piece of white paper in each color on the wall. Hold 

 a piece of red paper in each color. Hold a blue substance 

 in each color. Hold a black substance in each color. Hold a 

 lens in the color beam. 



Record your results and complete the following state- 

 ments. 



There are spectrum colors. The colors of the spec- 

 trum in the order of their appearance on the wall are . 



An object is white if it (reflects, absorbs) all the colors 



of the spectrum. An object is red if it reflects only the 



part of the spectrum. An object is black if it the 



colors of the spectrum. Placing the lens in the color beam 

 proves . 



OTHER INVESTIGATIONS WHICH YOU CAN MAKE 

 Examine various kinds of fabrics by means of sunlight. 

 Repeat the examination by light from an ordinary incan- 

 descent bulb; by any other sources of light. 



READINGS WHICH WILL HELP ANSWER 

 THE PROBLEM QUESTIONS 



What do scientists think light is? Scientists at the 

 present time are not certain that they know the exact 



nature of light. Sir Isaac 

 Newton, a great scientist of 

 the 17th century, thought 

 that light consisted of 

 streams of very tiny particles 

 or corpuscles, as he called 

 them, projected with rapid 

 speed from all luminous 

 bodies. Because of Newton's 

 great reputation in the field 

 of science this theory practi- 

 cally dominated scientific 

 thought for more than a cen- 

 tury. 



Today light is considered 

 to be a wave motion. The 

 question that arises, how- 

 ever, is, "What does this 

 wave motion travel in?" You learned in a previous 

 unit that sound is a wave motion in such things as air, 

 water and other substances and that sound waves 

 will travel only in solids, liquids, and gases. Light, 

 however, will travel across a vacuum. Light comes to 

 us from the sun, a distance of about 93,000,000 miles, 

 and most of the space which it traverses we believe 

 to be a perfect vacuum. Since we cannot conceive of 

 a wave motion without something to "wave," scien- 

 tists have found it necessary to assume a wave-trans- 

 mitting medium. This medium is called the ether and 

 it is assumed that it penetrates all space, even be- 

 tween the molecules of matter. The best definition 

 that we can make for light at the present time is that 

 light is a wave motion in ether. Even though this idea 



International News Photos 

 FIG. 155. SIR ISAAC NEWTON 



His best known achieve- 

 ment is the formulation of 

 the laws of gravitation. 



is being questioned today by some scientists, the 

 beginner in science will find it best to accept this as 

 a working definition until the nature of light is better 

 understood. 



What is the speed of light? Light travels at the tre- 

 mendous speed of 186,264 miles per second. This speed 

 is so enormous that up to the year 1675 it was thought 

 that light traveled instantaneously, that is, that no 

 time was consumed when 

 light passed from one 

 point to another. The 

 great scientist Galileo had 

 tried to measure the speed 

 of light as a beam of it 

 passed from one hilltop to 

 another, but was unsuc- 

 cessful. Roemer, a young 

 Danish astronomer, about 

 1675 was the first to dis- 

 cover the approximate 

 speed of light, and recent- 

 ly Professor Michelson, International News Photos 



who was professor of phy- FIG. 156. A. A. MICHELSON 

 sics at the University of Chicago, spent nearly a whole 

 life time making the most accurate determinations. 



The speed of light varies with the density of the 

 substance through which it is traveling. The denser 

 the substance the lower the speed. The speed of light 

 is greatest in a vacuum and it is greater in air than 

 in water or glass. 



Exercise. What causes shadows? See how much in- 

 formation you can secure about shadows by experiment- 

 ing. 



Suggestions: Use a darkened room, a bright source 

 of light, and a light screen such as a white wall, a piece 

 of white cardboard, or a white sheet. Place various ob- 

 jects in the path of the light beam. Can you find answers 

 to these questions? 



Which kinds of objects cast shadows and which do 

 not? 



Are shadows equally intense at all points? Hold a 

 cardboard in the light beam first near the screen and 

 then farther away from the screen to study this. 



Do shadows have depth, or are they only in the plane 

 of the screen? Record the results of this exercise in 

 your notebook. 



Light travels in straight lines. By common experi- 

 ences we know that it is impossible for us to see 

 a source of light when an object through which light 

 cannot pass is placed between our eyes and the source. 

 Also it is impossible to see around a corner. This is 

 because, under ordinary conditions, light rays travel 

 in straight lines through a given substance. This can 

 be shown by the experiments suggested at the be- 

 ginning of this topic. Have you tried them? 



