Radiant Heat and Light 167 



to throw back the red lights and keep all the other kinds 

 (absorb them and change them to heat), we can under- 

 stand what the dye in a red sweater does. The dye is 

 not really trained, of course, but for a reason which we 

 do not entirely understand, some kinds of dye always 

 throw back (reflect) any red that is in the light that shines 

 on them, but they keep all other kinds of light, changing 

 them to heat. Other dyes or coloring matter always 

 throw back any green that is in the light that shines on 

 them, keeping the other colors. Blue coloring matter 

 throws back only the blue part of the light, and so on 

 through all the colors. 



So if you throw a white light, which contains all the 

 colors, on a "red" sweater, the dye in the sweater picks 

 out the red part of the white light and throws that back 

 to your eyes (reflects it to you) but it keeps the rest of 

 the colors of the white light, changing them to heat; 

 and since only the red part of the light is reflected to 

 your eyes, that is the only part of it that you can see ; 

 so the sweater looks red. The " green" substance 

 (chlorophyll) in grass acts in the same way; only it 

 throws the green part of the sunlight back to your eyes, 

 keeping the rest ; so the part of the light that reaches 

 you from the grass is the green light, and the grass looks 

 green. 



Anything white, like a piece of paper, reflects all the 

 light that strikes it; so if all the colors (white light) 

 strike it, all are reflected to your eyes and the object 

 looks white. 



You have looked at people under the mercury-vapor 

 lights in photo-postal studios, have you not? The 



