162 



LIGHT AND SOUND 



heat (cf. 183), 



181. Absorption of Light; Color. Certain substances 

 do not allow light to pass through them, and do not 

 reflect it; what, then, becomes of the light? We say it is 

 absorbed (173). It is really changed almost entirely into 



Lampblack, or soot, is a good absorber 

 of light; silver is an almost perfect re- 

 flector (cf. 57 and 175; also Fig. 

 157). 



We see most objects by the light they 

 reflect (cf. 165 and 174). The color 

 of an object thus depends upon the color 

 of the reflected rays, or, in the case of a 

 transparent body, upon the color of the 

 light that passes through the body. A 

 red body sends red rays to the eye, and 

 absorbs all other rays. If it reflects 

 nearly all rays, it is light gray. If it 

 absorbs all, it is black. 

 Two colors that yield white light when mixed are called comple- 

 mentary colors. Such are red and blue-green, yellow and blue-indigo, 

 orange and light blue, green-yellow and violet. Color is not really a 

 property of bodies, but a sensation which they produce upon ourselves. 

 What we call color is the sensation that a body produces when we see it 

 in ordinary daylight. We all know how different the same colors ap- 

 pear in daylight, gas light, and the electric arc light. A blue ribbon will 

 not appear blue unless the light by which it is illuminated contains 

 blue rays. Daylight contains blue rays. But the light of sodium 

 vapor (cf. 109) contains only yellow rays. In such light all objects 

 are either yellow or black. For the same reason a blue object held in 

 the red light of a photographer's " dark room " appears black. 



182. The Sky and Its Colors. The normal color of the 

 sky is blue, except when the atmosphere contains too 



FIG. 157. 



Two Air Thermometers. 

 The air in the black- 

 ened bulb becomes 

 warmer than that in 

 the colorless, or sil- 

 vered, one; the black 

 bulb absorbs the more 

 light and heat. 



