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thickener t1_j8rw9kz wrote

This answer got me wondering, if a photon could somehow enter an atmosphere, pass through water, then head back into the vacuum of space, does it “re-accelerate” to “full” lightspeed? How does that work?

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Chemomechanics t1_j8s6uup wrote

No one thing is decelerating or accelerating. The photon–matter interaction propagates slower than c.

See the discussion here and the links within, for example.

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