ENERGY-CONSERVATION WITH PARTIALLY COHERENT SOURCES WHICH INDUCE SPECTRAL CHANGES IN EMITTED RADIATION

Some questions are examined that have been raised in connection with the recently discovered phenomenon of spectral changes induced by the state of coherence of sources. It is shown that even though the spectrum of light changes on propagation in free space, there is no violation of the law of conservation of energy throughout the space outside the source region and that, in fact, energy is conserved at each frequency. It is also shown by a simple example that even when a partially coherent or a fully coherent source is spherically symmetric it will, in general, produce a far field which, whilst also spherically symmetric, will have a spectrum that differs from the source spectrum; and that this result is also consistent with the law of conservation of energy.