The Lens and Focusing Light on the Retina - Video Tutorials & Practice Problems

Optics: Lenses and Refraction

All right 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 gonna 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 gonna 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 in 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 All right, we can use that to our advantage using lenses. So we're gonna 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 b bulging outwards instead of bulging inwards. And when we focus light, we always want to focus it on a surface. So here I put, put a surface on the screen here. And this surface well, 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 gonna 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 gonna use an R as an example. So if we have this letter R light is gonna hit the letter R and it's gonna bounce off in every which way, every possible direction some of that light is gonna bounce off and hit this lens as it hits this lens, it's going through a medium of a different density it's gonna bend and the convex lens is gonna 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 gonna 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. All right. Now, the thing about this, this works really well, but everything is at this very set distance. If I were to move the R move the lens or move the surface, it's not gonna be focused anymore. They're not gonna 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's two basic ways that you can just a focus a just 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 some, how some eyes work. We mentioned the octopus. I briefly uh 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 gonna say the shape of the lens affects how much light is bent, allowing us to focus at different distances, right? 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 gonna say the surface is still the same distance from the lens though. So a more convex lens or a rounder lens is gonna 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, it's gonna be moving 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 and 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 all the points on the R and we end up projecting this R. Now, one thing you'll notice this R because it's closer when you do all the physics out, it ends up being projected larger on the surface. And that's why things that are closer to you appear bigger. All right. Well, we can go in the opposite direction. We have a less convex lens here. You can see this lens looks very flat. Well, a less convex lens or flatter lens will mean less refraction. And our surface here is still the same distance. But now we're gonna focus on something that's farther away and this thing that's farther away when the light is bouncing off that thing that's farther away when the light that hits that lens is gonna be moving well, a little bit closer to parallel, it's not gonna be as much of an angle. Therefore, the lens doesn't have to bend it back quite as much. So we can see this all play out and we're gonna project this R onto the surface here. The light doesn't have to bend as much because it's coming back closer to parallel when it's further away. And when this all works out, you'll see that this R gets projected smaller because it's farther away. And that's why things that are farther away look smaller, they get projected smaller on your retina. All right. So that's the basics of physics of lenses. Now, we not need to talk about how does your lens actually physiologically change shape? We'll do that coming up. See you there.

The Lens and Focusing Light on the Retina Example 1

Our example here tells us that you're reading a book in the library, you look up to see a friend entering the room and how would you expect the lens of your eye to change shape in order to focus on your friend across the room? All right. So it's really just asking what's the shape of the lens while you're focusing on the book and then what's the shape of the lens when you're focusing on the student across the room? Well, if you're reading a book, you're probably looking at something close to your face, you have that close vision. And so what shape of the lens do we say it would be in to focus on something that is closer to you? Well, remember we said that when something is closer to you, your lens has to do more work, to refract the light, to focus it back onto the retina. And so it needs to take this more convex shape or I'll just write in parentheses, a rounder shape that has to sort of bulge out and be fatter. All right, your friend walks in the room, hey, buddy, across the room, you're looking at a distance. What shape is your lens gonna take? Then? Well, when you're looking at a distance, you want your lens to be bus convex or we put in parentheses flatter right. Light coming from a distance is hitting your eye kind of more in parallel where light close up is hitting at your eye at more of an angle. So this light coming at you from far away, your lens has to do less work to refract it and bend it back to reach a single point on your retina. And so it can be flatter to do that. All right. So remember that more convex, rounder, close vision, less convex, flatter. Further way vision, we haven't talked about how your lens changes shape yet that's coming up. I'll see you there.

True or False: if false, choose the answer that corrects the statement.

The lens of the eye focuses on nearer objects by becoming less convex.

• A. True.
• B. False, our eyes focus on nearer objects by moving the lens further from the retina.
• C. False, our eyes focus on nearer objects by changing the shape of the cornea.
• D. False, the lens of our eyes focus on nearer objects by becoming more convex.

Accommodation: Changing Focal Distance

As we talk about the lens and how we focus light on the retina. We now want to think physiologically what happens in the eye to do this and this process of changing our focus, we're gonna call accommodation, right? So accommodation is changing our focal distance and understand how this works. We first just wanna note that there's actually two structures in the eye that are gonna refract light. Well, first is the cornea, the cornea of that front of your eye that actually has the most refraction and but it's not flexible, right? So it bends light the most entering your eye. But because it's not flexible, it can't focus, you can't change the way it bends the light. So if we have our R here, it's gonna enter the eye, it's gonna refract at the cornea. But the thing that's gonna do the most work in focusing, that's gonna be the lens because the lens can change shape and that is what's gonna get things to focus on a really clear point giving you a really clear image when you look at things. Now, to understand how the lens works, you need to understand that the lens Well, we said that it's this flexible disk, that flexible disk is normally stretched to be flatter, right? So there's tension on all the way around on the outside of that lens, pulling it flatter, pulling it thinner. Now, remember a flatter lens that allows us to see things that are farther away. So at rest, your eye is just primed to see things that are farther away, to see things closer to you, to focus on things close up. Your eye has to do some work. So that's what we want to talk about. What is the work that your eye has to do to change that focus to allow you to see things that are close up. Well, we're gonna say to focus on near object, near objects, the eye actually uses multiple systems. So we're gonna talk about more than the lens right here, but we'll start talking about the lens. All right. So we're gonna call this lens accommodation. And we're gonna say in lens accommodation, the ciliary muscles contract when the ciliary muscles contract that releases tension on the lens. When the tension is released, the la that flexible lens lens, it bulges, it becomes more round or I'm gonna say here, more convex, right? We said a more convex or rounder lens is able to focus on things that are closer up. All right. So remember those ciliary muscles, a circular muscle surrounding the lens, the ciliary zones are suspending the lens in between there. And just to look at this, we're gonna zoom in now on this structure. So now we have this front view of the ciliary muscles, the ciliary zonal and suspending the lens in the middle. Now, the idea that a muscle contracts to release tension is sometimes a little counterintuitive. Normally think when a muscle contracts, it puts tension on something. But you see this ciliary muscle, it's a circular muscle. So when it contracts, it squeezes in like a sphincter and the space on the inside gets smaller, it's connected to those ciliary zones. So as it squeezes in, it pulls on the ciliary zones less and less. So we can see that here, this muscle is gonna squeeze in and those CIA zones are gonna get a little bit of slack in them that allows that lens to bulge out because there's no longer tension on it as it bulges out, it becomes rounder and you're focusing on things that are closer up now to see something farther away. Again, the opposite happens those lenses or I'm sorry, those muscles just relax. That puts the tension back on the lens, it flattens out. All right, I'm gonna move this out of the way just so I have more room here, but we'll leave that up there for a reference. Ok. So that's lens accommodation, that's what happens with your lens. The next thing that your eye needs to do is called the accommodation, pupillary reflex. So your pupil just like that ciliary muscle is going to constrict. Ok. So the muscle and the iris is gonna contract make those pupils smaller, that's gonna block light from hitting the edges of the lens. Now, remember when you're looking at something closer, that thing that is closer to the light is coming at your eye at more of an angle. So it's more likely to hit the edges of the lens. The edge of the lens doesn't focus as well as the center of the lens. So to block that the pupil just closes down a little bit and it keeps the light hitting the center of the lens. So it's able to focus nicely. Now you see something farther away, the opposite happens, it just opens up and that light is hitting your eye more straight on. So it's not gonna be hitting the edge of the lens nearly as much as if you're looking at something close up. You don't need to worry about it as much. OK. The final thing that we're gonna do here, we're gonna call eyeball convergence and this you're probably familiar with, right? It just means that as you look at something as it's closer and closer to your face, your eyes cross and that is to focus the image on the phobia, right? That phobia, that center of your vision on the retina to keep that image focused on the phobia of both eyes, they need to cross to keep things focused everything in the center of your vision. All right. The last thing I want to note here as people get older, their lenses become less flexible. Now, that's true of a lot of things in the body, right? As folks get older, things become less flexible, they don't bend as well, they don't spring back as well. True for the lens. So the lens is normally stretched flat. So if it becomes less flexible, it no longer bulges out, we're gonna say that it loses its ability to accommodate. So that means as you get older, you lose your ability to see things that are close up. And this happens to everyone, roughly around age 50 you see older people, they try and look at their phone, they start holding it farther and farther away from themselves or they put on their readers, right? No shame in asking help from your friends gonna happen to all of us. I'll see you in the next video.

The Lens and Focusing Light on the Retina Example 2

Our example here says that the two images below represent cross sections of the same eye when focused at two different focal lengths based only on the images. Can you determine which eye is focused on something close and which is focused on something far? We want to draw arrows to two structures that are different between the images that lead you to make your conclusions and note the changes shown are exaggerated for the purposes of illustration. This isn't exactly the scale. All right. So we have two eyes here that look very similar. We have this transverse section, this top down view of the right eye. And first thing we wanna do, just take a second. What do you see that is different between these two images? All right. As I look at them, I see two things I see first, the lens, this lens on the eye on the left looks really round, almost marble shaped where this lens on the right looks much flatter, more disc shaped. The other thing I see the size of the pupil. This one has a really small pupil that iris is squeezed in making the pupil small over here. The pupil is big that Iris is pulled back more. So I'm just gonna label those things. I'm gonna say the lens is more convex here and over here the lens is less convex. I'm gonna say here we have a small people and over here we have a large people. All right. So when I think of a small pupil and a more convex lens is that for close vision or far vision, well, that's for looking at something close. When I look at a large pupil and a less convex lens, well, then that's something looking at something that is far away. All right. So just to look at this in a little bit more detail, remember when something is close to the light that's scattering off, it is hitting your lens at more of an angle. And so this lens has to do more work to bend it back to make that clear image on the retina. So you need a rounder lens to do that. Now, just real quickly, one thing that's not shown very well here is that these sary muscles should be contracting and squeezing inwards. And that's what's actually gonna release that tension on the lens and allow it to bulge out. But of course, now, in contrast, we can look at the far distance here at far distance light is hitting the lens closer to parallel. So the lens doesn't have to do as much work to bend it back and make that clear image on the retina here, these ciliary muscles would be relaxed. Therefore, exerting pre or, or pulling on those ciliary zones and pulling that lens flatter. Now, for the pupil here again, because this light is coming at more of an angle. You wanna block the light from hitting the edges of the lens because the edges of the lens don't focus as well as the center of the lens does for far vision. Again, this lights coming straight on, you don't need to block it from hitting the sides of this lens nearly as much. Ok. So remember just sort of at rest, we're focused at things far away and we call the process of focusing on something close accommodation. Practice is more and practice problems coming up. I'll see you there.

Two of the processes involved in accommodation for near vision involve contracting a muscle. In which answer choice below is the muscle matched with the correct form of accommodation?

• A. Ciliary muscles—Lens accommodation.
• B. Ciliary muscles—Accommodation pupillary reflex.
• C. Pupillary dilator—Accommodation pupillary reflex.
• D. Pupillary constrictor—Lens accommodation.

In a previous example, you saw that when reading a book, your lens is rounder or more convex, but when you look up to see a student entering the library, your lens becomes flatter or less convex. What muscle action results in the lens becoming less convex?

• A. Ciliary muscles contract.
• B. Ciliary muscles relax.
• C. Pupillary constrictors contract.
• D. Pupillary dilators relax.