Alright let's ask you a question: why can't you focus underwater? Okay, let's say you go swimming, right? Underwater and you're not wearing a mask. Your're just floating around down here. Here's your big old eyeballs you're having a good time. And you're trying to see this fish but it looks all blurry, okay? The fish looks blurry. You have a hard time focusing underwater. Why is that? To understand this we need to think about what's happening to the rays of light that are coming from the fish to our eyeballs. Okay, and let's draw a couple pictures here. Here's your eyeball. And when parallel rays come in, they are focused to a point at the back of the retina. Okay? At the back of your eyeball at the retina. But let's draw it in a little more detail to see what happens in this bending at the cornea. This is the cornea of your eyeball and when this parallel ray comes in, it's going to hit this first interface. This is air out here. This is your cornea as the rest of your eyeball. And so there's going to be bending at that first surface. The light is going to bend at the cornea. And it's that bending at the cornea that's going to allow it to focus on the back of your eyeball, but why does it bend there? Alright. It bends because Snell's law says n-air sine-theta, air equals n-cornea sine-theta cornea. And there is a different index of refraction between the air and the cornea and that means that theta has to change. Okay? It's going to bend towards the normal since the cornea is curved. Those parallel rays will bend inward towards their relative normals . But what happens when you're underwater? When you're underwater, here come the light rays in, here's our cornea. There's our eyeball and now we're not in air anymore we are in water. And so Snell's law says that n of water sin theta of water equals n of the cornea sin theta of the cornea. And so it's going to bend if those are different and it's not going to bend if they are the same. And what happens is those are nearly the same. N water is nearly the same as n of the cornea. And therefore, theta in the water is nearly the same as theta in the cornea. Things don't focus. And so you can't see under water. You can't focus very well under water. Now, anybody know why this would be? Why would your cornea have nearly the same index of refraction as water? Any ideas? Because you guys are made up of water, okay? You are mostly water and so the cornea has nearly the same index as water because a lot of it's made of water, right? Sort of this gelatinous stuff but it's the same idea, okay? So how do you fix this problem? When you're swimming under water and you do want to see those fish, how do you fix the problem? You get an air water interface again by putting on a mask. And by putting on that mask you put a little air in front of your eyeballs. And when the light rays come now and they go to the air cornea interface they can bend and come to a focus. Alright? This is the whole reason for the mask. is just to provide a little air next to your eyeballs. In fact you don't need a mask if there was another way to provide air right there, you can do it, and you've seen this before, right? If you take a big bucket and put it over your head and you go underwater and now there's an air gap between the water level and your eyeballs, then you can see just fine again.