Question

29 In 1849, Fizeau carried out the first terrestrial measurement of the speed of light; previous measurements by Roemer and Bradley had involved astronomical observation. The figure shows a simplified conceptual representation of Fizeau's experiment. A ray of light from a bright source was directed through the teth at the edge of a spinning cogwheel. After traveling a distance L, it was reflected from a mirror and returned along the same path. The figure shows the case in which the ray passes between two teeth, but when it returns, the wheel has rotated by half the spacing of the teeth, so that the ray is blocked. When this condition is achieved, the observer looking through the teeth toward the far-off mirror sees it go completely dark. Fizeau adjusted the speed of the wheel to achieve this condition and recorded the rate of rotation to be f rotations per second. Let the number of teeth on the wheel be n. (a) Find the speed of light e in terms of L, n, and J (b) Check the units of your equation using the method shown in example 1 on p. 22. (Here f's units of rotations per second should be taken as inverse seconds, sI, since the number of rotations in a second is a unitless count.) (c) Imagine that you are Fizeau trying to design this experiment. The speed of light is a huge number in ordinary units. Use your equation from part a to determine whether increasing e requires an increase in L, or a decrease. Do the same for n and f. Based on this, decide for each of these variables whether you want a value that is as big as possible, or as small as possible (d) Fizeau used L 8633 m, f 12.6 s-1, and n # 720, Plug in to your equation from part a and extract the speed of light from his data. to Problem 29. Prtsn 6

Answer 1

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The main confusion arises at high speed rotation when the light rays are blocked by the tooth of the wheel by half of the time..think about it..