160 LIGHT AND SOUND 



its composition than colored light; but this is not so. If 

 a beam of sunlight is admitted through a narrow slit (A) 

 into a dark room (Fig. 155), an image of the opening will 

 be made upon the screen at B. But if a glass prism (P) 

 is put in the path of the sunbeam, the image of the slit 

 will not be a simple line, but a series of lines side by side; 

 that is to say, a band. The image will not be white, but 

 will consist of bands of different colors. As the prism is 



placed in Fig. 155, the 

 violet band is highest, 

 while the red band is 

 lowest. The reason is 

 that violet light is bent 

 most by the prism, and 



A glass prism breaks up white sunlight into red, least. The Order of 

 light of seven well-marked colors. .. , ... , . 



the colors will be: violet, 



indigo, blue, green, yellow, orange, and red. The initial 

 letters make the fanciful word VIBGYOR. 



When platinum, iron, etc., are heated, they first become red hot, and 

 last of all white hot. This suggests that we get the red rays first because 

 they have the slowest rate of motion. Only when the temperature has 

 risen are the violet rays added which are necessary to give pure-white 

 light. The red rays have the longest waves; the violet waves are 

 shorter, but move much more rapidly. 



APPROXIMATE LENGTH OP LIGHT WAVES, IN MILLIMETERS 



Violet 000397 Yellow 000589 



Indigo 000431 Orange 000656 



Blue 000480 Red 000688 



Green 000527 



If a reversed prism is placed so as to catch the colored 

 bands produced by the first prism (Fig. 156), the colors 



