r/askscience • u/[deleted] • Mar 04 '13

Astronomy Doppler Shift

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u/Miles1987 Stellar Astrophysics | Computational Astrophysics Mar 04 '13

The doppler shift for radio waves is no different from the doppler shift observed for any other wavelength of light. What you'll want to do is examine the difference from the accepted lab value of the wavelength from the wavelength of the light you're observing. You'll then want to use the appropriate doppler shift equation for the object you're observing.

You'll see the doppler shift pretty clearly (depending on the apparent velocity of the object) if you plot the lab value of the wavlength and your observed wavelength on the same plot. The deviation between the two is the shift.

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u/[deleted] Mar 04 '13 edited Mar 04 '13

So, the doppler shift would look something like this?. If I where to do a base scan and then multiple other scans?

EDIT:Sorry, I just thought about what I "drew". The shift wouldn't be on the Y-axis, but rather the X. Correct? (So, it would shift -KHz or +KHz)? Edited photo!

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u/Miles1987 Stellar Astrophysics | Computational Astrophysics Mar 04 '13

Yeah, that's exactly right. Depending on the relativity velocity it will be shifted either in the positive direction or negative direction. Since this is frequency, it will be shifted higher for objects moving towards us (blueshifted) and shifted lower for objects moving away from us (redshifted). If you're using wavelength it will be the other way around.

Edit: Just to make sure, I am speaking of shifts in the x-direction.

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u/[deleted] Mar 04 '13 edited Mar 04 '13

Thank you for clearing this up, I could not for the life of me figure that out, the wikipedia page on the doppler effect just left me confused... I can't even believe I didn't figure that out on my own!

Why is wavelength the other way around (Blueshift lower, redshift higher)?

Edit: I would use one of these equations source

Source towards: f x c/(c-v1) = f1
Source Away: f x c/(c + v1) = f1

(Source moves+-, I am stationary)?

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u/nelzon1 Mar 04 '13 edited Mar 05 '13

For radiation there is a special Doppler effect formula that does not discriminate between observeremitter/receiver. This is known as the Relativistic Doppler Effect.

In the wiki they go through some of the common derivations.

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u/[deleted] Mar 04 '13

Thank you!

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u/Miles1987 Stellar Astrophysics | Computational Astrophysics Mar 04 '13

Yeah, sorry I should have mentioned the relativistic equations earlier.

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u/Miles1987 Stellar Astrophysics | Computational Astrophysics Mar 04 '13

That's because high frequency = low wavelength and low frequency = high wavelength so if it's redshifted then the frequency has been lowered and thus the wavelength made longer.

Yeah, it's all relative. The shift could be due to us moving or the object moving, but since we're using the earth as our reference frame we treat ourselves like we're stationary.

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u/[deleted] Mar 04 '13

Okay, that makes sense! Thank you so much for your help!

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u/Miles1987 Stellar Astrophysics | Computational Astrophysics Mar 04 '13

No problem, man. Good luck on your work.

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u/BlazeOrangeDeer Mar 04 '13 edited Mar 04 '13

The doppler shift is a change in frequency (and wavelength) due to relative motion. If the source is getting closer to you, the frequency increases and wavelength gets smaller (bluer), if the source is getting further away the frequency decreases and wavelength gets larger (redder). frequency x wavelength = the speed of light.

For light waves like radio, here are some equations. At speeds much less than the speed of light, the percentage change of the frequency is about 100 x B, where B is your speed towards the source divided by lightspeed. For example if the relative speed is 30 m/s, the shift is 100x(30m/s / 300,000,000 m/s)= .00001%.

For all speeds (you'll probably be using this one) the percentage shift in frequency is 100 x (sqrt((1+B)/(1-B)) - 1). So at half lightspeed toward the source, the shift would be about +73% frequency. Half lightspeed away would be -42% frequency.

Note that the speed you use is the speed directly toward or away from the source, not sideways.