According to classical light theory the energy in an electromagnetic wave is proportional to the amplitude of the wave squared (A²). Doesn't doppler effect disprove this idea ? Picture an oscillating charge going up and down, emitting two waves in opposite directions perpendicular to the oscillation. The two waves will have the same energy, equal in amplitude and wavelength. Now, if we were to apply a doppler effect to this oscillation, what would happen ? The emitted energy on each side would stay the same, as would the amplitude. The wavelength on the other hand, would increase in one direction and decrease in the other. Well, all is fine then - The energy is proportional to the amplitude since it does not change and the wavelength does.... ... right ? .. Wrong. Spatial distribution is not accounted for in this conclusion. As a result of the doppler effect, the energy in the shorter wavelength wave is now distributed in less space, which gives a higher amount of energy per unit "volume space", which I believe is the property actually measured. Two observers comparing measurements of the two waves would argue that the energy is proportional to the frequency, since the energies and frequencies mismatch equally much, and the amplitudes are the same. My conclusion is that, in classical light theory, the energy in an electromagnetic wave should be proportional to the amplitude squared and the frequency. E = kA²f ?