Hi, Poly.<quoted text>
Actually, this isn't quite right. The crucial aspect isn't the delay in the re-emission. It is the *phase shift* between the absorbed photon and the emitted one. In most media, the phase shift depends on the frequency. It turns out that a phase shift is completely equivalent to saying the velocity is different. This is why in anomalous dispersion we get 'speeds' of photons being faster than light in a vaccum. The phase shift is opposite the usual one, so instead of being equivalent to a slower speed, we get a shift equivalent to a faster speed.
In this view, it actually happens that light *always* moves at c, but phase shifts make it *appear* that it moves faster or slower.
My little run-down wasn't completely wrong, though... was it.? It was just incomplete?
Though the delay between absorption and emission isn't the crucial aspect... it is an aspect.(?)
Light's *apparent speed* varies across a medium composed of matter and empty space... as everything we consider as something consists of a lot of nothing... the speed through the empty space alone remains constant.
Isn't phase shift of a wave of given frequency still dependent upon what it travels into?
Is it okay that my thinking is derived from what happens as electrical current flows into a capacitor? Electrons accumulate on the negative plate and are pushed from the positive plate. Potential builds across the plates... but, only after current begins to flow through the device. Thus, current flow is said to lead voltage through a capacitor......
...... in the same way fluid flow leads pressure across a tank. Something must flow into the tank before pressure builds. It's the restriction of flow at the tank's exhaust that gives rise to pressure.
I know I'm mixing analogies in that I'm referring to two different *electrical* characteristics as they apply to an electrical device... namely current (electrical flow) and voltage (electrical pressure)... and not sticking with one or the other. Signal phase at the output of a composite circuit can appear to lead that at the input when scoping only current or voltage (ie: dual-trace oscilloscope).
I'm also comparing electrical flow to fluid flow... and trying to extend that to something more fundamental.
Could something *similar* be said for a photon when one encounters an atom?
Not super-specifically... but, just as a general arm-chair explanation?