It is shown that the Kramers-Kronig formula used in reflection spectroscopy remains unproved for real materials. Stern’s proof makes use of a very simple, not necessarily realistic model of a solid. Two possible justifications for using the Kramers-Kronig formula on “well-behaved” materials are noted. Some misconceptions about the general relationship between the amplitude and phase of a causal linear response are also pointed out.
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Ralph H. Young (1977) studied this question.
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