 We've seen that flat and curved mirrors can be used to manipulate images, but a more common device to do that is the lens. Lenses can occur in nature, for example, in the eye. The surface of the eye has a lens, which is made of transparent cells. Remember, the light travels slower in materials, so if you have a large amount of light coming into the eye, then when the light hits the middle of it, it's going to slow down, and the light at the edges is going to go faster, so it's going to wrap around and come together. If you build the lens right, so you have the right thicknesses varying as you go across, you can have parallel rays of light coming into one point called a focus, and that's called a converging lens. And you can work out how images work for converging lenses in exactly the same way you do for mirrors. So if we have a focus for a lens, and we have to have a focus on either side, because light rays could go either way through a lens, then if you have an object on one side, you can figure out what kind of image you're going to get by following the normal rules of light rays. A light ray that goes through a focus will go out parallel, and a light ray that goes through parallel will go through a focus. So you can say that these two rays appear to be coming from something back here, and so in this case we've got a virtual image of someone. Alternatively, if you have someone standing outside the focus, you get a different kind of image, but you work it out the same way. Parallel rays go through the focus, and rays that go through the focus come out parallel. And so here you can see that the light rays really seem to be coming from this point. So that's where the top of the head image is, and that is a real image, and an inverted one. So you can see the way you work this out is the same every time. Remember, these things are called converging lenses. You can also build diverging lenses. A diverging lens has more material on the outside and less on the inside, like this. What that means is that light rays are going to go the other way. The ones that come in parallel are going to spread out. And if you build them just right, you can make it so that they all seem to spread out from a particular point, which is also called the focus. And because lenses are symmetric, you can have light go either way, you have a similar focus on the other side. And you can also make images of diverging lenses following exactly the same rules. So here we have our object, and if we have a light ray that goes in parallel, it will seem to come out from this focus. And if we have a light ray that's coming out parallel on this side, then it has to be as though it was going through this focus. And so we can see that the two rays on this side appear to be coming from a point back here. And so we have a virtual image down here. And once again, we're just following exactly the same rules to work out where our images are. And we can do that no matter where the object is in relation to the focus.