Alright, folks. As we start talking about light and lenses and how we focus that light on the retina, we need to know a little bit about physics. Now don't worry, this is not a physics class. There's going to be no numbers here. You just really sort of need an intuitive sense about how lenses work. So when we talk about the physiology of the eye, you understand what it's doing and why it's doing it. So we're going to start out just by saying that light refracts. And by refracts, we just mean it bends when it passes between media of different densities. And this is something I'm sure you're familiar with. If you've ever been at a swimming pool and you look at something under the water, you realize the thing that you're looking at isn't exactly where you know it to be. Same thing if you look at a straw and a glass. Right? As that straw enters the water, it seems to bend or be in a different place than where you know it is. That's because the water and the air have different densities. So as light passes from one to the other, it bends. Alright. We can use that to our advantage using lenses. So we're going to say that a convex lens is used to refract light in a way that allows it to be focused. And remember, convex means that it bulges outwards, so we have an example of a convex lens here. It's rounded, it's bulging outwards instead of bulging inwards. And when we focus light, we always want to focus it on a surface. So here I've put a surface on the screen here, and this surface will in a camera that surface might be the film. If you're projecting something to watch a movie that surface would be the screen. In your eye, that surface is going to be the retina. Now because our retina is curved on the back of the eye, I've made this surface sort of curve to sort of mirror our retina in that way. Now we're trying to focus something, focus some image and here we're going to use an 'r' as an example. So if we have this letter 'r', light is going to hit the letter 'r' and it's going to bounce off in every which way, every possible direction. Some of that light is going to bounce off and hit this lens. As it hits this lens, it's going through a medium of a different density, it's going to bend. And the convex lens is going to take this light that is spreading out from the 'r' and bend it back so it meets again at a point. And if we put our surface at the right distance, it's going to meet at a point on that surface. So we can take a look here, we pick a random point, light scatters, it hits that lens and it gets bent back and it gets focused on this point on the surface. We can choose another point, do the same thing. We can do it for all the infinite points of the 'r' and we end up projecting that 'r' onto that surface. Now the thing you'll notice, that 'r' is now upside down and backwards. So we're going to say that an image produced by a convex lens will be inverted, upside down and backwards. Now that means that the image that's projected on your retina is going to be upside down and backwards from how things are in the real world. Now that kind of blows some people's minds that your brain takes this image that's upside down and backwards and flips it all around so it doesn't look that way anymore. I'll just say your brain does a lot of complex stuff, but that is one of them. Alright. Now the thing about this, this works really well, but everything's at this very set distance. If I were to move the 'r', move the lens, or move the surface, it's not going to be focused anymore. They're not going to hit at that perfect point. So we look at things that are different distances away, and so we need to be able to adjust our focus. Now there are 2 basic ways that you can adjust a focus, adjust focus using a lens. You could move the lens back and forth, and that's how a camera works and that's how a projector works and that's somehow how some eyes work. We mentioned the octopus eye briefly, previously. That's how an octopus eye works, It focuses by moving the lens back and forth. Not what we do. We change the shape of our lens. OK? So we're going to say the shape of the lens affects how much light is bent, allowing us to focus at different distances. Alright? And that shape, we can basically make it more convex, we can make it rounder and fatter, or we can make it less convex, we can make it flatter. So let's take a look at how this works. First, we'll look at this more convex lens here. So you can see this lens, it's rounder than the first one that we looked at. And we have this surface. We're going to say the surface is still the same distance from the lens, though. So a more convex lens or a rounder lens is going to lead to more refraction. And so if we look at this 'r', that means that you can focus on an object that is closer to you. Now this object that's closer to you, when the light is scattering off it, the light that hints hits the lens will be moving at more of an angle than it was if it's a little bit further away. This means that this rounder lens needs to bend that light back even more to reach the same point. So it has to do more work of bending, and that's what a rounder or more convex lens can do. So we can see this play out. Right? You can see it's hitting that lens at a greater angle, but the bigger curve is able to focus but back. Same thing, we do a...
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15. The Special Senses
The Lens and Focusing Light on the Retina
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