Transformation of Electromagnetic Amplitude


Section 7 of Einstein’s 1905 paper on the electrodynamics of moving bodies considers an electromagnetic plane wave propagating in vacuum. Since the amplitudes of the electric and magnetic field vectors of a plane wave are equal, it follows that in terms of any standard system K of inertial coordinate x,y,z,t the components of those vectors can be written in the form



where A is the amplitude of each vector, and the phase angle F is given by



The symbols ex, ey, ez are the direction cosines of the electric field vector, i.e., they are the cosines of the angles between the electric vector E and the x, y, and z axes respectively.  Likewise the symbols bx, by, bz denote the direction cosines of the magnetic field vector B, and the symbols nx, ny, nz denote the direction cosines of the wave normal vector n (i.e., the unit vector pointing in the direction of the wave propagation).


We know the three unit vectors n, e, and b are mutually orthogonal, as are unit vectors in the x, y, and z, directions, so we have the relations



We also note that n = e x b, so the components of n can be expressed as



Obviously the squared amplitude of the electric field oscillation is the sum of squares of the components Ex, Ey, Ez at their maximum values, so we have



and similarly for the magnetic field vector.


Now, in terms of another standard system K’ of coordinates x’,y’,z’,t’ moving relative to K in the positive x direction with speed v, the components of the electric and magnetic field vectors are





where w’, nx’, ny’, and nz’ are the frequency and direction cosines of the wave normal in terms of the K’ system of coordinates. These are given by the Doppler formula



and the aberration formulas



Einstein concluded Section 7 of his paper by saying “We still have to find the amplitude of the waves as it appears in the moving system”. He then says “we obtain” the formula for the ratio of squared amplitudes, but doesn’t gives the actual calculation. As in the unprimed system K, the squared amplitude in the primed system K’ is equal to the sum of squares of the components Ex’, Ey’, Ez’ at their maximum values, so we have



Dividing through by A2 and factoring out g2, we get



Expanding the right hand side, this gives



Making use of the identities



the equation reduces to



Since nx = cos(f) where f is the angle between the wave normal and the x axis, this gives the result as it appears in Einstein’s paper



Comparing this with the Doppler formula for the frequency, we see that A’/A = w’/w, meaning the amplitude of an electromagnetic plane wave is strictly proportional to the frequency. Now, from Poynting’s Theorem we know the energy density is proportional to the square of the amplitude, so we might think the total energy of a given portion of a plane wave is proportional to the square of the frequency. However, as Einstein showed in Section 8 of his paper, the volume of a given “light complex” is inversely proportional to the frequency (consistent with the relation lw = c), and hence the total energy is proportional to frequency.


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