It is well known that the speed of light depends on the index of refraction of the medium in which the light is propagating. It is also well known that in a dispersive medium, the speed of an amplitude modulated wavefront depends on the group refractive index, i.e., slightly slower than the carrier light. Corrections for the group refractive index in air are typically made for temperature, humidity, and pressure—without which measurements could be in error by tens of parts per million. The internal instrument optical elements are also subject to dispersive effects, which have heretofore been ignored in the literature—and presumably in the design. Note that this is probably because no commercially available optical design software package models amplitude modulated wavefronts. A thought experiment will illustrate the problem.

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