The observer moves away from the stationary light source with speed v:

https://www.youtube.com/watch?v=SC0Q6-xt-Xs

The speed of light the moving observer measures is either variable, c'=c-v, or invariable, c'=c.

Einstein and Feynman fraudulently teach that, if the moving observer measures the speed of light to be variable, c'=c-v, the principle of relativity is violated:

Albert Einstein: "If a ray of light be sent along the embankment, we see from the above that the tip of the ray will be transmitted with the velocity c relative to the embankment. Now let us suppose that our railway carriage is again travelling along the railway lines with the velocity v, and that its direction is the same as that of the ray of light, but its velocity of course much less. Let us inquire about the velocity of propagation of the ray of light relative to the carriage. It is obvious that we can here apply the consideration of the previous section, since the ray of light plays the part of the man walking along relatively to the carriage. The velocity W of the man relative to the embankment is here replaced by the velocity of light relative to the embankment. w is the required velocity of light with respect to the carriage, and we have w = c - v. The velocity of propagation of a ray of light relative to the carriage thus comes out smaller than c. But this result comes into conflict with the principle of relativity set forth in Section V." http://www.bartleby.com/173/7.html

Albert Einstein, On the Principle of Relativity: "After all, when a beam of light travels with a stated velocity relative to one observer, then - so it seems - a second observer who is himself traveling in the direction of the propagation of the light beam should find the light beam propagating at a lesser velocity than the first observer does. If this were really true, then the law of light propagation in vacuum would not be the same for two observers who are in relative, uniform motion to each other - in contradiction to the principle of relativity stated above." https://einsteinpapers.press.princet.../vol6-trans/16

Richard Feynman: "Suppose we are riding in a car that is going at a speed u, and light from the rear is going past the car with speed c. Differentiating the first equation in (15.2) gives dx'/dt=dx/dt-u, which means that according to the Galilean transformation the apparent speed of the passing light, as we measure it in the car, should not be c but should be c-u. For instance, if the car is going 100,000 mi/sec, and the light is going 186,000 mi/sec, then apparently the light going past the car should go 86,000 mi/sec. In any case, by measuring the speed of the light going past the car (if the Galilean transformation is correct for light), one could determine the speed of the car. A number of experiments based on this general idea were performed to determine the velocity of the earth, but they all failed - they gave no velocity at all." http://www.feynmanlectures.caltech.edu/I_15.html

The argument used by Einstein and Feynman is invalid, but physicists are unable to see the invalidity. They would not even think in this direction:

"Crimestop means the faculty of stopping short, as though by instinct, at the threshold of any dangerous thought. It includes the power of not grasping analogies, of failing to perceive logical errors, of misunderstanding the simplest arguments if they are inimical to Ingsoc, and of being bored or repelled by any train of thought which is capable of leading in a heretical direction. Crimestop, in short, means protective stupidity." http://ebooks..adelaide.edu.au/o/orw...hapter2.9.html

Pentcho Valev