London retardation wavelength

The electric field involved in London interactions propagates with the speed of light from one particle (molecule, ion, or colloid) to another, if the particles are far apart, there is a time retardation (equivalent to a phase lag) between the vibrations of both the bodies. The importance of the retardation increases as the distance approaches the wavelength of the electric field, A. 

The retardation correction to the London attraction betwee n flat plates is found by introducing the retardation correction to the attraction of two atoms ( 9c, p. 266) in the considerations of the preceding subsection. In 9c the retardation correction has been given in terms of 2tc r/x (x r- London-wavelength). When the distance between the plates is much larger than X, only that part of the London attraction which varies... 

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