Distance-dependent dissipation

If light pays an energy toll to the medium over cosmic distances, a fair question follows. Why does it not lose energy quickly when forced through something dense, like a glass lens?

RSI answers by separating two effects. The first is speed: the atoms of a lens raise its refractive index and slow light while it is inside. The second is Distance-Dependent Dissipation: the energy toll depends only on how far the wave travels against the 0.73 N Universal Field Tension. Over each stretch of path, every wave gives up the same fraction of its frequency. Losing amplitude would only dim light; losing frequency shifts it toward red, and because every wave loses the same fraction, all the lines in a galaxy's spectrum shift by the same factor, as observed. Over billions of light-years, that adds up to cosmological redshift.

A lens spans a negligible distance, so the toll paid inside it is tiny. Slowing a wave does not consume its energy, so the light leaves the lens at its original frequency.

One part of the account remains open. Distant supernovae brighten and fade more slowly than nearby ones, by the same factor as their redshift. Expansion explains this automatically. RSI must show that the medium's toll stretches the timing of light signals by the same factor.