# How does light accelerate back to its original speed after refraction from another medium e.g. glass or water

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The speed of light slows as it traverses through a different medium, but after slowing it finds a way to accelerate back to original speed as it exits that medium. How does it do that?

In: 93

Something to keep in mind that any individual photon of light isn’t slowed down. But the medium absorbs and reemits the light causing an overall slowdown.

Think of it like taking a football and passing it through a crowded room. You can’t throw it through people so they have to catch the ball and throw it again. But get it the edge of the crowd and yeet it through open space there’s no more delay from catching and throwing again.

A photon/wave of light doesn’t require an energy source once it’s created. It exists as pure energy and it is its own energy source so it will always go back to its expected speed after it passes through some medium that “slows” its progress.

I have no idea, but what goes through my mind right now is: Photons don’t have mass, therefore they don’t need any energy to accelerate since F = m*a.

The speed of light does not actually change at all, it only appears to. A common misconception is that photons are absorbed and re-emitted, but this is wrong. The real explanation is quite hard to ELI5, but I’ll give it a go.

It starts by understanding that light behaves a bit like ripples in a pool. As these ripples pass through stuff, the stuff jiggles a bit. But the jiggling stuff causes it’s own, slightly delayed ripples. When you add these ripples to the original ripples it looks like one ripple moving slower. So we say “light moves slower” even though the original ripples and the delayed ripples travel at full speed.