$$ \frac {E'}{E} = (\frac{1 - \frac{v}{c}}{1 +
\frac{v}{c}})^{\frac{1}{2}} $$
It is to be noticed that the energy and the frequency of a light-complex
vary according to the same law with the state of motion of the observer.
Let there be a perfectly reflecting mirror at the co-ordinate-plane ξ =
0, from which the plane-wave considered in the last paragraph is
reflected. Let us now ask ourselves about the light-pressure exerted on
the reflecting surface and the direction, frequency, intensity of the
light after reflexion.
Let the incident light be defined by the magnitudes A cos Φ, _v_
(referred to the system K). Regarded from _k_, we have the corresponding
magnitudes:
$$ A' = A \frac{1 - \frac{v}{c} \cos \Phi}{\sqrt{1 - \frac{v^2}{c^2}}}
$$
$$ \cos \Phi' = \frac{\cos \Phi - \frac{v}{c}}{1 - \frac{v}{c} \cos
\Phi} $$
$$ \nu' = \nu \frac{1 - \frac{v}{c} \cos \Phi}{\sqrt{1 -
\frac{v^2}{c^2}}} $$
For the reflected light we obtain, when the process is referred to the
system _k_:—