Doppler effect (moving observer):

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

Two obvious facts in the above scenario:

1. The wavelength (distance between light pulses) is constant.

2. The frequency and the speed of the light pulses vary proportionally for the moving observer.

One might say that the two facts are only relevant for this particular scenario, but no, they are universally valid. Generalised over all possible scenarios, the two facts become, respectively, the fundamental axiom and the fundamental law of future, Einstein-free physics:

The wavelength of light is constant (for a given emitter).

Any frequency shift entails (is caused by) a proportional speed-of-light shift.

It is easy to see that the fundamental axiom and the fundamental law of Einstein-free physics are obeyed he

University of Illinois at Urbana-Champaign: "Consider a falling object. ITS SPEED INCREASES AS IT IS FALLING. Hence, if we were to associate a frequency with that object the frequency should increase accordingly as it falls to earth. Because of the equivalence between gravitational and inertial mass, WE SHOULD OBSERVE THE SAME EFFECT FOR LIGHT. So lets shine a light beam from the top of a very tall building. If we can measure the frequency shift as the light beam descends the building, we should be able to discern how gravity affects a falling light beam. This was done by Pound and Rebka in 1960. They shone a light from the top of the Jefferson tower at Harvard and measured the frequency shift. The frequency shift was tiny but in agreement with the theoretical prediction. Consider a light beam that is travelling away from a gravitational field. Its frequency should shift to lower values. This is known as the gravitational red shift of light." https://courses.physics.illinois.edu...re13/L13r.html

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