If Φ = 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 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:
For the reflected light we obtain, when the process is referred to the system k:—
A″ = A′, cos Φ″ = -cos Φ″, ν″ = ν′
By means of a back-transformation to the stationary system, we obtain K, for the reflected light:—
