Don't think about individual photons. Think about billions of them with destructive and constructive interference. The probabilities of all the sitting l additive waves of light.
That's light as an aggregate wave. Photons, actual light, always travel at c. What's happening in a medium is the rapid absorption and readmission of photons. The probability of admission is based on structure of material causing things like lens or mirrors to work.
You can think of it as the photons having to jump between platforms before the can continue running at c.
Speediest little fella. (mander.xyz)
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