Page:Light waves and their uses.djvu/76

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58

��Light Waves and Their Uses

��ferometer, with a corresponding increase in the delicacy of the test.

I will conclude by showing how to measure the length of light waves by means of the interferometer. By turning

���FIG. 53

the head attached to the screw, one of the interferometer mirrors (namely C, Fig. 39) can be moved very slowly. This motion will produce a corresponding displacement of the interference fringes. Count the number of interference fringes which pass a fixed point while the mirror moves a given distance. Then divide double the distance by the number of fringes which have passed, and we have the length of the wave. Using a scale marked from to 10, made of such a size and placed at such a distance that, when a beam of light reflected from a mirror attached to the screw moves over one division, a difference in path of one-

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