= Relativistic Doppler effect
{title2=$\nu=\gamma\nu'(1+\beta\cos\theta')$}
The <Lorentz transformation> changes the <photon> <four-momentum> and therefore its frequency, producing the displayed formula for a boost along the first axis. Here the primed ray angle is measured in the emitting frame and $\beta=v/c$. The angle and frequency transforms must be used together: <aberration of light> changes the direction, and a photon-number angular fraction need not equal the corresponding radiant-energy fraction.
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