Refraction in a glass of water | Waves | Middle school physics | Khan Academy

So, something very interesting is clearly going on when we look at this pencil dipped in this cup of water. We would expect if maybe there was no water in this glass that we would just see the pencil continue straight down in a line that looks something like that. But that's clearly not what we are seeing. It looks like once we fill it with water, at least to our eye, it looks like the pencil gets bent or broken or bent in some way.

This notion you might have heard people call it refraction, but it's interesting to think about exactly why this is happening. And I'll give you a hint: this is all about the bending of light. And it's not just light that can get bent as it goes from one medium into another; it can be any kind of wave.

So, let's think about what's going on over here. First, let's think about the part of the pencil that is above the water, so this part right over here. The light is actually reflecting off of this pencil, and then it's bouncing straight into our eye. So, just imagine a path from this dot straight into your eye. Once again, from here it's going straight into your eye.

When we go over here, it still doesn't look too distorted. So, you have light that's going straight to your eye; it's going through the side of the glass and then getting to your eye. But then once we get into the water, something's interesting happens. You would expect the point that what would have been here would then go straight to your eye, just like everything up here.

But it turns out that that light, once it transitions from going from the water to the glass and then the air, it bends. So, at the interface between the media, between those different materials that the light is traveling through, instead of going towards your eye, it gets bent, in this case to our left.

And so, that's why when we look straight on here, we don't see anything in this region right over here. But the light that was going from the pencil towards this part of the glass, which typically you would not see—that would have typically just gone straight in that direction and not hit your eye—well, now that is getting refracted. It's getting bent to the left so that now that light hits your eye.

So, that's why you see what looks like a broken pencil. It's all about the light getting bent as it exits the water and goes into the glass and then the air.