# When does the observed wave velocity differ from the actual wave velocity in the Doppler effect?

In some explanations to the formulae of the doppler effect, and also in answers to certain questions, the approach taken is to look at the observed velocity. In such approaches, the observed velocity is treated as being different to the actual wave velocity. However, in a real situation, the conclusion that I have come to after hours of thought is that frequency (hence period) and wavelength are the only variables which change. However, the observed wave velocity should equal the actual wave velocity.

So, am I right in coming to that conclusion?

Any help is appreciated :)

• Hi, could you please just check I edited your question title correctly, we try to keep titles to the keywords, and we avoid uppercase most of the time in the post. Thanks. – user108787 Sep 14 '16 at 13:41
• Thanks for the comment and your time man, but unfortunately I wanted the question to be worded the way it was because I genuinely believe that the observed velocity should be indifferent from the actual wave velocity. However, I didn't know about the no uppercase rule, so I'll keep that out. Thanks again man, I really appreciate the help :) – Mathematician Sep 14 '16 at 13:46
• Actually CountT010, I think you were right in rewording the question now that I've thought about it even more. Thanks man :) – Mathematician Sep 14 '16 at 13:52
• Speed of light (or any electromagnetic wave) is always measured as the same "c" in a vacuum, regardless of moving towards or away from the source. It's not true for other waves like sound. – Peter R Sep 14 '16 at 14:01

That depends on the type of wave, as @Peter R commented. For light, the basic point of special relativity is that you will always observe the same speed of light $c$, no matter whether you move towards or away from the source or whatnot (there is no "medium" to define a rest frame).