*View: https://www.youtube.com/watch?v=bg7O4rtlwEE*

The fact that the motion of the observer does not change distances between subsequent pulses is obvious. Moreover, this fact is universally used, explicitly or implicitly, in the derivation of the Doppler frequency shift (moving observer):

Max Planck Institute for Gravitational Physics: "You can see for yourself that, once more, there is a blue-shift - the pulse frequency measured at the receiver is somewhat higher than the frequency with which the pulses are sent out. This time, THE DISTANCES BETWEEN SUBSEQUENT PULSES ARE NOT AFFECTED, but still there is a frequency shift." https://www.einstein-online.info/en/spotlight/doppler/

"Thus, the moving observer sees a wave possessing the same wavelength...but a different frequency...to that seen by the stationary observer." http://farside.ph.utexas.edu/teaching/315/Waveshtml/node41.html

"Let's say you, the observer, now move toward the source with velocity Vo. You encounter more waves per unit time than you did before. Relative to you, the waves travel at a higher speed: V' = V+Vo. The frequency of the waves you detect is higher, and is given by: f' = V'/λ = (V+Vo)/λ." http://physics.bu.edu/~redner/211-sp06/class19/class19_doppler.html

"Vo is the velocity of an observer moving towards the source. This velocity is independent of the motion of the source. Hence, the velocity of waves relative to the observer is c + Vo...The motion of an observer does not alter the wavelength. The increase in frequency is a result of the observer encountering more wavelengths in a given time." http://a-levelphysicstutor.com/wav-doppler.php

The only problem with the obvious fact referred to above is that it is fatal for Einstein's relativity. If the motion of the observer does not change distances between subsequent light pulses, then the speed of the pulses relative to the moving observer VARIES proportionally to the frequency at the moving observer.

In this thread I am going to show that the generalization of the obvious fact, which is

The wavelength of light is constant.

is universally valid. And if so, "The wavelength of light is constant" deserves to become the fundamental axiom of future, Einstein-free physics.