Red-shift

unfinished

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If you stand by a road-side and count the cars passing by in a given time, you will soon be bored.  If you now drive along the same road, and your passenger counts the cars you overtake, and the cars which pass you on the other side of the road, you will find that the first rate is smaller, and the second rate is larger, than the original rate.

The same sort of things happens with sound waves.  When an ambulance passes you, the sounds that you hear as it approaches and as it recedes are noticeably different.  There is a sharp drop in pitch as the vehicle passes.