In the video below, the speed of light is obviously variable (frequency and speed of light pulses vary proportionally for the moving observer), and the wavelength (distance between pulses) is obviously constant:

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

If the emitter (not the observer) starts moving, things are less obvious. Einsteinians teach that the wavelength of light varies with the speed of the emitter, but this is absurd (contradicts the principle of relativity). The wavelength is constant in that case (moving emitter) as well.

When light falls in a gravitational field, the frequency and the speed vary proportionally, and accordingly the wavelength remains constant. This is clearly shown 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

The fundamental axiom of future (Einstein-free) physics: The wavelength of light is constant (for a given emitter).

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