Why does light change speed?

Light slows down when it travels through things like water or glass because it has to work harder to get through those materials.

Imagine you are running down an empty hallway. You can go super fast because there is nothing in your way. This is like light traveling through empty space, called a vacuum. Now, imagine that same hallway is filled with people standing still. You have to weave between them. You cannot run in a straight line. You have to bump into them, stop, and start again. This extra effort makes your average speed much slower.

Light does the exact same thing. When light enters water or glass, it meets tiny particles called atoms. The light waves push against these atoms. The atoms absorb the light and then send it back out. This constant stopping and starting makes the light seem to move slower overall.

This change in speed is why a straw in a glass of water looks broken at the surface. The light bends, or changes direction, when it moves from air into water. We call this refraction. It is not magic; it is just physics doing its job.

So, light slows down because it is interacting with the material it is passing through. More stuff in the way means more slowing down.