CHAPTER XXIX 



SOME EFFECTS OF LIGHT. PHOTOGRAPHY 

 PHOTOSYNTHESIS 



Cause of the Spectrum. You have learned that the 

 rainbow is due to the separation of sunlight into its rays 

 of different wave-lengths. You have also noticed that 

 rainbows appear only when the sun is low enough in 

 the sky that the effects produced by fine drops of water 

 are reflected back to us from the other side of the heavens. 

 Now that you have studied refraction, you can understand 

 why drops of water may cause the breaking up of light 

 into its spectrum. Each kind of ray is refracted in a slightly 

 different manner from the other kinds, dependent upon the 

 nature of its wave-length. Then, if there be behind the 

 refracting surface another surface that catches and reflects 

 back to us the rays that have thus become separated into 

 their own kinds, our eyes receive the image of a spectrum. 



You have seen rainbows in the spray of a fountain or 

 lawn-sprinkler. The drops of water, just as in the case of 

 a rainbow in the sky, act both as refractors and as reflectors. 



Also you can see why a glass prism may produce a 

 spectrum. Note that in the spectrum produced by a 

 prism in the manner shown in the figure the colors occur 

 in the following order, reading upward: red, orange, 

 yellow, green, blue, indigo, and violet. This is because 

 the red rays, whose wave-lengths are longest, are refracted 

 least, while the violet rays, whose wave-lengths are shortest, 

 are refracted most. This order of the colors of the spec- 

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