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Q & A: speedy car colors

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Most recent answer: 10/22/2007
Q:
The doppler effect talks about moving objects creating a red or blue shift... and this is why a car will sound different comming toward you then going away from you (asuming you are not going the same speed as the car) What I want to know is if your car went fast enough would not only the sound but the color of the car seem to change to an observer? If so about what speed might this occour at?
- James
A:
Sure, light from the car will be Doppler shifted. That will be pretty evident once the car is travelling at least a few per cent of the speed of light, relative to you. Say 10^4 km/s, or around 2*10^7 mph might be enough. You'd have to go off-road.

Much smaller Doppler shifts are easily measurable with instruments, if not obvious to the eye. Some speed checks are done using Doppler instruments, and these obviously must resolve changes of 1 mph or so to be useful. These don't use the smear of colors coming off a car in sunlight but rather use a very narrow range of frequencies from some source included in the speed detector.

Mike W.

(published on 10/22/2007)

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