Page:SahaElectrodynamics.djvu/26

 moving system, which are enclosed by the surfaces mentioned above, then

If $$\Phi=0$$, we have the simple formula :—

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 $$\xi=0$$, 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 $$A cos \Phi$$, v (referred to the system K). Regarded from k, we have the corresponding magnitudes :