The Lens and Focusing Light on the Retina: Study with Video Lessons, Practice Problems & Examples

Light refracts when passing through different media, allowing lenses to focus images on the retina. A convex lens bends light to project images, which appear inverted. The eye adjusts focus through accommodation, where ciliary muscles contract to change the lens shape, allowing for clear vision of nearby objects. The pupil constricts to enhance focus by blocking peripheral light, while eyeball convergence aligns both eyes on the fovea. Aging reduces lens flexibility, leading to presbyopia, making it harder to see close objects.

Optics: Lenses and Refraction

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...

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? Alright. 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. It has to sort of bulge out and be fatter. Alright. Your friend walks in the room. Hey, buddy. Across the room, you're looking at a distance. What shape is your lens going to take then? Well, when you're looking at a distance, you want your lens to be less convex. Or we'll put in parentheses, flatter. Right? Light coming from a distance is hitting your eye 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. Alright. So remember that more convex, rounder close vision, less convex, flatter, farther away 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 about what happens in the eye to do this. And this process of changing our focus, we're going to call accommodation. Alright. So accommodation is changing our focal distance. And to understand how this works, we first just want to note that there are actually two structures in the eye that are going to refract light. Well, first is the cornea. The cornea, the front of your eye, actually has the most refraction, 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 light here, it's going to enter the eye. It's going to refract at the cornea, but the thing that's going to do the most work in focusing, that's going to be the lens. Because the lens can change shape, and that is what's going to 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. Alright? So there's tension 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? But we're going to say to focus on near objects, the eye actually uses multiple systems. So we're going to talk about more than the lens right here, but we'll start talking about the lens. Alright. So we're going to call this lens accommodation. And we're going to 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 flexible lens bulges. It becomes more round, or I'm going to say here more convex. Right? And we said a more convex or rounder lens is able to focus on things that are closer up. Right. So remember those ciliary muscles, a circular muscle surrounding the lens. The ciliary zonules are suspending the lens in between there. And just to look at this, we're going to zoom in now on this structure. So now we have this front view of these ciliary muscles, the ciliary zonules, and suspending the lens in the middle. Now the idea that a muscle contracts to release tension is sometimes a little counterintuitive. Normally, I 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 zonules, so as it squeezes in, it pulls on the ciliary zonules less and less. So we can see that here. This muscle is going to squeeze in, and those ciliary zonules are going to 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. Alright. I'm going to move this out of the way just so I have more room here, but we'll leave that up there for a reference. Okay. 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. K? So the muscle and the iris is going to contract, make those pupils smaller. That's going to block light from hitting the edges of the lens. Now remember, when you're looking at something closer, that thing that is closer, 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 if 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 going to 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 as much.

The final thing that we're going to do here, we're going to 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 fovea. Right? That fovea, that center of your vision on the retina, to keep that image focused on the fovea of both eyes, they need to cross to keep things focused, everything in the center of your vision. Alright. 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. It's true for the lens. So the lens is normally stretched flat, so if it becomes less flexible, it no longer bulges out. We're going to 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. It's going to 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 to scale. Alright. So, we have two eyes here that look very similar. We have this transverse section, this top-down view of the right eye. And the first thing we want to do, just take a second. What do you see that is different between these two images? Alright. 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 disk-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 going to label those things. I'm going to say the lens is more convex here, and over here, the lens is less convex. I'm going to say here we have a small pupil, and over here we have a large pupil. Alright. 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, that's for looking at something that is far away. Alright. So, to look at this in a little more detail, remember, when something is close to you, 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 ciliary muscles should be contracting and squeezing inward, and that's what will actually 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, thereby exerting or pulling on those ciliary zonules and pulling that lens flatter. Now for the pupil, here, again, because this light is coming at more of an angle, you want to block the light from hitting the edges of the lens because the edges of the lens do not focus as well as the center of the lens does. For far vision, again, this light is coming straight on. You do not need to block it from hitting the sides of this lens nearly as much. Okay. So remember, just sort of at rest, we're focused on things far away. And we call the process of focusing on something close accommodation. We'll practice this 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.

Here’s what students ask on this topic:

How does the lens of the eye focus light on the retina?

The lens of the eye focuses light on the retina through a process called accommodation. The lens is a flexible, convex structure that can change its shape to focus light. When viewing distant objects, the ciliary muscles relax, causing the lens to flatten. For near objects, the ciliary muscles contract, releasing tension on the lens, allowing it to become more convex. This increased curvature bends the light more, focusing it precisely on the retina. The retina then receives the inverted image, which the brain processes and flips to perceive the correct orientation.

What is the role of the cornea in focusing light?

The cornea plays a crucial role in focusing light as it enters the eye. It is the transparent, curved front surface of the eye that provides most of the eye's refractive power. When light passes through the cornea, it bends significantly due to the change in medium from air to the denser corneal tissue. However, the cornea is not flexible and cannot change its shape, so it cannot adjust focus. Instead, it provides a fixed amount of refraction, while the lens fine-tunes the focus to ensure a clear image on the retina.

What is accommodation in the context of the eye?

Accommodation in the context of the eye refers to the process by which the eye adjusts its focus to see objects at different distances. This is achieved by changing the shape of the lens. When focusing on near objects, the ciliary muscles contract, reducing tension on the lens, allowing it to become more convex. This increased curvature bends the light more sharply, focusing it on the retina. For distant objects, the ciliary muscles relax, increasing tension on the lens, making it flatter and reducing the amount of light bending, thus focusing the image on the retina.

Why do images appear inverted on the retina?

Images appear inverted on the retina due to the way light is refracted by the convex lens of the eye. When light rays from an object pass through the lens, they are bent in such a way that they converge at a point on the retina. This bending causes the top of the object to be projected at the bottom of the retina and vice versa, resulting in an inverted image. Similarly, the left side of the object is projected on the right side of the retina and vice versa. The brain then processes this inverted image and flips it to match the actual orientation of the object.

How does aging affect the lens's ability to focus?

Aging affects the lens's ability to focus due to a condition called presbyopia. As people age, the lens becomes less flexible and loses its ability to change shape easily. This reduced flexibility means the lens cannot become as convex as it needs to for focusing on close objects. Consequently, older individuals often find it difficult to see things up close and may need reading glasses to compensate for this loss of accommodation. This typically begins to affect people around the age of 50.