Good day to you all, I wanted to share a struggle with you all that I can't seem to get out of. It's about calculating, in the case of sound, the decrease or increase in frequency (thus increase or decrease in wavelength) when a sound source is respectively moving away or towards an observer with a velocity using the formula z = v / s (where s is the speed of sound). I know that this formula is only accurate for small velocities. Scenario in which a sound source is moving away from the observer: In this case, I'd automatically conclude that the observed wavelength (λObs) would be larger than the emitted wavelength (λEmit). The factor the emitted wavelength gets larger with would be the ratio of of the speed of sound (s) divided by its speed minus the velocity of the object that is moving away (v). λEmit x (s / (s - v)) = λObs The frequency would be inversely lower by a factor of (s - v) / s fEmit x ((s - v) / s) = fObs If one would want to calculate λEmit from the observed wavelength and the velocity of the object, the formula would look like this: λObs / (s / (s - v)) = λEmit The part (s / (s - v)) could be rewritten as 1 / (1 - (v / s)) (divide all parameters by s). Since z = v / s, then I could rewrite that part as 1 / (1 - z). Thus, the formula would be: λObs / (1 / (1 - z)) = λEmit This means that: λObs / λEmit = 1 / (1 - z) In principle, the z in this formula would be considered the redshift in the case of light, since the object is moving away from the observer. Here's when thought I had the formula right, until I read the formula on the wiki page here: https://en.wikipedia.org/wiki/Redshift which shows that the formula should be: λObs / λEmit = 1 + z Instead of my concluded formula: λObs / λEmit = 1 / (1 - z) My question is, what the heck am I doing wrong here?