The observer starts moving relative to the emitter in Doppler:

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

The wavelength (or distance between light pulses) obviously remains constant. Accordingly, the frequency and the speed of the pulses VARY proportionally for the moving observer, in violation of Einstein's relativity.

The emitter starts moving relative to the observer in Doppler:

https://www.youtube.com/watch?v=xsVxC_NR64M

It is universally taught that the wavelength of light varies with the speed of the emitter, as shown in the video, but this contradicts the principle of relativity. If the wavelength varied, the emitter would measure its varying value, inside his spaceship, and so he would be able to calculate his spaceship's speed without looking outside. The wavelength of light is constant, independent of the speed of the emitter. Accordingly, the speed of the light and the frequency, as measured by the stationary observer, VARY with the speed of the emitter, in violation of Einstein's relativity.

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