# Doppler Effect on Light: Constant Speed of Light Confusion

• I
• InquiringMind
In summary, the speed of light in a vacuum is the same for all observers. This is why there is a Doppler effect, which is a shift in the frequency or wavelength of a wave.
InquiringMind
TL;DR Summary
Doppler effect and constant speed of light
If the speed of light in the vacuum of empty space is the same for all observers, how can there be a Doppler effect on light? Doesn't the shift of color/frequency indicate a change in speed?

Sorry if I posted with the wrong prefix, I'm just a curious senior (as in old guy)

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InquiringMind said:
TL;DR Summary: Doppler effect and constant speed of light

If the speed of light in the vaccum of empty space is the same for all observers, how can there be a Doppler effect on light? Doesn't the shift of color/frequency indicate a change in speed?

Sorry if I posted with the wrong prefix, I'm just a curious senior (as in old guy)
No, it indicates a change in frequency and wavelength such that the product, which is equal to the speed, stays constant.

InquiringMind said:
TL;DR Summary: Doppler effect and constant speed of light

If the speed of light in the vaccum of empty space is the same for all observers, how can there be a Doppler effect on light? Doesn't the shift of color/frequency indicate a change in speed?

Sorry if I posted with the wrong prefix, I'm just a curious senior (as in old guy)
No - if the wavelength and frequency for one observer are ##\lambda## and ##\nu## and another observer measures wavelength ##\lambda'=\alpha\lambda## then they measure frequency ##\nu'=\nu/\alpha##. Then ##\lambda'\nu'=\alpha\lambda\nu/\alpha=\nu\lambda## so the wave speed is the same.

This is qualitatively similar to sound waves in a medium but the mathematical details are different.

I just read that in a vacuum, all colors travel at the same speed, so I guess that is why there can be an observed color shift without a change in speed.

Dale
InquiringMind said:
I just read that in a vacuum, all colors travel at the same speed, so I guess that is why there can be an observed color shift without a change in speed.
As others said, it is the frequency shift. Here are some animations about the Doppler effect:
https://www.einstein-online.info/en/spotlight/doppler/

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Sagittarius A-Star said:
No. As others said, it is the frequency shift. Here are some animations about the Doppler effect:
https://www.einstein-online.info/en/spotlight/doppler/
Frequency shift and color shift are the same thing. Hence terms such as "red shift" and "blue shift".

Sagittarius A-Star
InquiringMind said:
TL;DR Summary: Doppler effect and constant speed of light

If the speed of light in the vacuum of empty space is the same for all observers, how can there be a Doppler effect on light? Doesn't the shift of color/frequency indicate a change in speed?

Sorry if I posted with the wrong prefix, I'm just a curious senior (as in old guy)

Just to make sure it's clear...
the Doppler effect is a relationship among three velocities:
• the velocity of the source
• the velocity of the receiver
• the velocity of the periodic signal (e.g. light or sound)

Longitudinal doppler shift is basically just time dilation in combination with the fact that the light pulses travels longer and longer distance towards the receiver

jbriggs444 said:
Frequency shift and color shift are the same thing. Hence terms such as "red shift" and "blue shift".
Yes, I should have mentioned this.

InquiringMind said:
I just read that in a vacuum, all colors travel at the same speed, so I guess that is why there can be an observed color shift without a change in speed.
Light moves in vacuum with the invariant speed and dispersion is not possible in vacuum, all colors travel at the same speed.

An observed color shift (Doppler effect) exists if the light moves in vacuum and also, if light would be sent through a dispersive medium. There, not all colors travel at the same speed. However, in this case the Doppler effect would be more complex.

You find a general treatment of the most simple case of light propagation in a dielectric and the Doppler effect here:

https://itp.uni-frankfurt.de/~hees/pf-faq/rela-waves.pdf

With "most simple case" I mean I consider only a monochromatic wave in a lossless medium with normal dispersion, i.e., concerning the optics the medium is described by a constant index of refraction, ##n>1##, i.e., the phase velocity of the wave ##c_{\text{s}}=c/n<c##.

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