 Suppose I have two regions in a wave medium where the speed is different and the wave is not coming straight on at that boundary edge. So suppose you have a boundary edge like this and waves travel at one speed on one side and a different speed on the other side. But the wave is not coming straight on. If the wave was coming straight on, then if the speed went up, then this wavelength would just go up. And if the speed went down, the wavelength would just go down. But what happens if it comes into an angle? Well, let's look at it. So remember, the wavelength over on this side has to be smaller. Which means these crests have to be closer together. You can imagine what would happen if you were in a line of people along a line and then at one point, a whole bunch of them went into a slow area. Maybe you were walking and they walked into mud or something and started going really slow. What would happen is the line would have to change angle. And you can see if we change the angle like that. So this is assuming that the speed over here is slower. If we change angle like that, then what we've got is that the wavelength here is smaller as we need it, than the wavelength over here. If these lines were straight, then obviously the wavelength would be exactly the same. So that angle has to change. So the wave has to change direction. So when you're going through a boundary between two different speeds, what you find is that the wave changes direction and that's called refraction.