# Doppler Effect

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How fast must we be approaching a distant galaxy emitting yellow light (590nm) for it to appear green (520nm)?

I believe it uses doppler effect, but how?

• Doppler Effect -

for non-relativistic speeds, we can use the approximation.

c/520 = (1+v/c) * c/590
520(1+v/c) = 590
1+v/c = 59/52
v/c = 7/52
v = 7/52 c

Is that relativistic? Generally, less than 0.1c is considered non-relativistic, so we may be a bit off, but if that matters, use the exact formula, which you can find with a quick web search.

Not being sure just how the Doppler effect is calculated, I just did some online searches. You could also have done this.

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