Apparatus for generating focused electromagnetic radiation

摘要

The fact that the intensity of the pulse decays more slowly than predicted by the inverse square law is not therefore incompatible with the conservation of energy, for it is not the same wave packet that is observed at different distances from the source: the wave packet in question is constantly dispersed and reconstructed out of other waves. The cusp curve of the envelope of the wavefronts emanating from an infinitely long-lived source is detectable in the radiation zone not because any segment of this curve can be identified with a caustic that has formed at the source and has subsequently travelled as an isolated wavepacket to the radiation zone, but because a certain set of waves superpose coherently only at infinity.