The Kramers opacity law has for bound-free and free-free absorption under its idealized ionized-gas assumptions.
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Kramers' opacity law, introduced by the physicist Hendrik Anthony Kramers, relates to the behavior of light as it interacts with matter, particularly in the context of the absorptive properties of materials. Specifically, Kramers' opacity law describes how the opacity (or the degree to which a material can block or absorb light) varies with the frequency of light and the parameters of the material.