Posted by **Fernandez** on Sunday, June 8, 2014 at 10:27am.

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 -
**Steve**, Sunday, June 8, 2014 at 11:14am
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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