February 4, 2011

030 How Eyes Work – An Introduction

In this video, Leslie explains all about how we are able to see with our eyes.


Transcript of Today’s Episode

Hello and welcome to another episode of Interactive Biology TV, where we’re making biology fun! My name is Leslie Samuel. In this episode, Episode 30, I’m going to talk about how eyes work. I’m basically just going to give a little bit of an introduction to how eyes work and how we see, how vision takes place. Before I get into it, I want to give a shout-out to Sarah and the other students in Brain and Behavior at the University of Windsor. They are, right now, going through this content and she sent me a question coincidentally right before I was about to get into this content. So thanks for the questions, and I hope in the next week, I’ll be able to answer those in the videos that I’m going to be doing.

Before I talk about how the eyes work, let’s recap on Episode 29 where we spoke about how senses work. We spoke about the 5 senses, and we said that the general mechanism, the general overview in terms of how it works is: first, receptors are stimulated, then signals get sent to the brain, and then the brain interprets the signals that it’s receiving. This is the way you smell, the way you see, the way you feel and so on. So that’s the general overview. Now, let’s get into the specifics of vision.

Now, imagine this with me. You’re a college student, let’s say you’re a guy and you’re walking on campus, and all of a sudden, out of the corner of your eye, you see a very attractive young lady. She’s so attractive that she causes you to turn your head and take a full look, and you’re just looking at this beautiful lady in admiration. Let’s talk about what happens.

Here we have your eye. This may or may not look like your eye, but it’s going to work for the purposes that I need it for right now. So how this works is there’s got to be light. If there’s no light, you’re not going to see. Let’s say here we have the sun that’s shining, or this light source can be a light bulb, or anything of that sort. But let’s say the sun is shining, and the sun is sending off photons of light. The photons of light are coming in contact with this individual. It’s coming in contact with her hair, with her skin, with her eyes, with all of the parts of her body. As it comes in contact with her body, light is also being reflected, and that light that’s being reflected enters your eyes.

Now let’s look a little closer at this eye. Here we see that we have an opening. That opening is called the pupil. That is a result of what we call the iris, so that’s this section right here, what gives you your eye color. So this iris has pigment that makes the eye appear to be blue. I think this is pigment, at least, I’m not sure if it’s a contact lens or not. But let’s assume this is the natural color of the eye. So the pigment is going to cause it to appear blue.

Now, have you ever noticed that if you go outside and it’s really bright, you end up having to squint? But then after a while, you can see and it’s fine. The reason that happens is because here we have the iris that can dilate. Basically, what can happen is we can adjust the size of this hole. If it’s too bright outside, this will get smaller so that it doesn’t let as much light in. If it’s darker, you come inside a room or anything of that sort, that’s going to expand and the pupil is going to be larger. Now let’s move on from there and see what happens next.

So now we’re looking at your eye, and here you see once again, we have the pupil. Here, you can see iris, and here you can see we have a lens. Now, the cool thing about it is we have also this ciliary muscle here, and that can contract. What that basically does to the lens is it can expand the lens or make it so that you can focus the light onto the retina, and you can see the retina is over here.

So we want to focus on the retina, which is this layer of tissue to the back of the eye, and it goes all the way around. We want to focus the light onto that because in the retina, we have 2 types of pigments, and the pigments are rods and we have cones. You’ve probably heard of these before. The rods are what help us in black and white vision, and the cones help us with color vision. Of course, they work together, but the cones allow us to see color, it allows us to see fine detail. However, the cones aren’t as sensitive to light as the rods. So the rods are really sensitive to light. It doesn’t take much light in order for the rods to be activated, but it does take more in order for the cones to be activated.

You can think about it this way. If you are looking at someone, and let’s say you’re still looking at that young attractive lady and the sun is shining. Now, what happens when the sun goes down? You start to see a little less detail, it gets darker so you can’t see as much detail as you would if there was a lot of light outside. That’s because when you have less light, yes the rods are definitely being stimulated and they’re responding, but the cones need more light in order for you to see bright colors, in order for you to see fine detail. So you lose some detail as it gets darker in the room.

So we have the rods and cones in the retina, and the place where we have the most pigment would be in the fovea. We have especially a lot of cones in this area.

So let’s go back to the scenario we were at before, where you’re looking at this attractive young lady, and light is being reflected off that lady, and it is entering your pupil. It comes in here, it goes through the cornea. It enters your pupil, and it comes to the lens. Okay, I’m going to draw 3 rays of light coming in here.

As I showed you before, the ciliary muscles that are located up here and down here, of course, those can contract and cause the lens to focus the light exactly how you want it to be focused. Now, since we said that we have most of the pigment, especially the cones, we have most of the cones in the fovea, what we want to do then, as this light comes in, you want to contract the ciliary muscles in such a way that the lens focuses the light onto the retina in a way that you can see the details of that attractive young lady.

So the ciliary muscles contract and that causes the lens to then focus the light directly onto the retina, where you have all the rods and the cones, especially in this fovea area where you can see a lot more detail. That is where you want the light to be focused so that you can see that attractive lady, or if you’re reading a book, you want to be able to see the letters and you want to see all the details of the letters. So we want to focus that in the area where we have the maximum amount of cones possible.

Now, we’re going to talk a lot more about what happens inside the rods and cones in a future episode, but right now, I just want to give you a general overview. Light comes in, lens focuses the light onto the retina, then when that happens, that causes a signal in the optic nerve. That signal, of course, goes towards the brain, and as we showed before, the brain then interprets that signal.

Let’s say this process isn’t working very well. Let’s say there’s something wrong with the lens here, where it’s not doing its job at focusing the light onto the retina. What can you do about that? Well, what you can do about that is something that is done very often, somehow compensating for the lens not working well by wearing glasses. So what I’m going to do is, and I’m going to act as if, and forgive my drawing skills here, I’ve never drawn glasses before. But with glasses, what I’m doing is I am putting a lens in front of the eye that can help so that when the light comes in, it gets focused how I want it to be focused, and then I can see the detail that I couldn’t see because of how my lens is compromised.

So there’s going to be a lot of detail in terms of how you create these lenses, and of course, it’s going to vary from individual to individual, but the idea with glasses is we want to make it so that, although the lens that’s inside our eye might not be doing a great job at focusing the light onto the retina in the right way, we can compensate for that by adding an artificial lens and then that can help us out in that process.

That’s really it for this video. It gives us an overview of how vision works and how the eyes work. In a future episode, as I mentioned, we’re going to go in to much more detail, some of the nitty-gritty about how this works even on a molecular level, some of the proteins that are involved and how this really causes a signal in the optic nerve to go to the brain, and what happens specifically in the brain. That’s it for this video, and I’ll see you in the next one.

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