# physics

what would the speed of an observer be if a red(4.688x10^14Hz) traffic light appeared green(5.555x10^14Hz) to the observer?
a) 2.219x10^8 m/s
b) 4.445x10^8 m/s
c) 5.548x10^7 m/s
d) 8.438x10^7 m/s
e) 2.890x10^6 m/s

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1. Use the approximate Doppler formula, for V/c <<1, which is

(delta f)/f = V/c
and see which V on the list is closest.

The exact Doppler formula for light is bit more complicated. If you know it, use it. Usually, it is only taught when you study the theory of relativity. It is NOT the same as the formula for the Doppler shift of sound heard by a moving observer.

(Delta f)/f = 0.185
V = 5.551*10^7 m/s

Go with (c)

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posted by drwls

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