Obviously variable (Newtonian) speed of light in Doppler effect:

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

The speed of the light pulses as measured by the stationary observer is

c = df

where d is the distance between the pulses and f is the frequency measured by the stationary observer. The speed of the pulses as measured by the moving observer is

c'= df' c

where f' f is the frequency measured by the moving observer.

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