We all know that light bends when it travels through glass, water, or other transparent material

that's how a microscope, lighthouse and spectacles work!

And you might even know that light bends

because it travels slower through glass or water than through air

but why does light slow down?

And how does it speed up again when it comes out on the other side?

There's nothing there to give it a push!

Well, if you think light is a wave, it's easy to explain

electromagnetic waves simply travel slower through glass than through air.

So the wave crests are closer to each other,

but the light still oscillates the same number of times per second, it stays the same color.

When the wave hits the air again, its color stays the same,

while its crests spread out and it returns to light speed.

The simplified explanation is that the energy of a wave is determined by its frequency, or color, which doesn't change!

So it doesn't need a boost to speed up at the other side.

But wait, you say!

I thought light traveled at the same speed in every reference frame

you still haven't explained how it can slow down!

Well, now let's think about light as a particle.

When light goes through glass, it gets knocked around and bumps into all sorts of molecules and electrons.

So whenever it's traveling, it's traveling at the speed of light

but it's busy interacting with and scattering off lots of stuff along the way,

and it doesn't necessarily take the shortest path through the glass.

It's like the President of the United States trying to cross a room

if the room is empty, he can walk across directly.

But if the room is full of people all of whom want to shake the President's hand,

even though he walks from person to person at full “Presidential-speed”,

he'll get slowed down along the way.

As soon as he reaches the far side of the room, though, he's free to resume his pace.

Full speed ahead, Mr. President!