The apparatus used by the French scientist Armand Fizeau in 1849 for measuring the speed of light is illustrated in Fig. 26.19. Teeth on a rotating wheel periodically interrupt a beam of light. The flashes of light travel to a plane mirror and are reflected back to an observer. Show that if the wheel is rotated at just the right frequency f, the light passing through one gap reaches the mirror and is reflected to the observer through the very next gap. When f is adjusted so this happens, show that the speed of light is given by c = 2fNL, where N is the number of gaps in the wheel, f is the frequency (in revolutions per second) made by the rotating wheel, and L is the distance between the wheel and the mirror.

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