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Structure of human eye

Let's explore the major parts of our eye, and see what each one does.  Created by Mahesh Shenoy.

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Video transcript

- Our eyes enable us to see all the beautiful things around us. In this video, we're going to see the structure of this eye, the things that make up our eye. When we look at our face, it looks somewhat like this, that's my poor, attempted drawing of face, but if you were to concentrate only on the eye and get rid of all the skin, then you would be left with a ball. And that ball is usually called the eyeball. Now, let's look at this eyeball from the side and let's assume it is transparent so that we can see inside. Here it is. So, we'll look at the different major parts of our eyes, and we'll also see what each one does. Let's start with the one that Is most visible to us, this one. This structure. This is called the iris. The iris. And though people talk about the color of the eye, they're actually talking about the color of the iris. So, when light hits our eyes, the light that falls on our iris gets reflected, and the color of the light that gets reflected totally depends upon the pigment that makes it up. In this case, it is green, and so you would say that person has green eyes. In my case, for example, it is brown, so my iris reflects brown light, and so my eyes are brown in color. But, notice our iris has a hole in it's center. That hole is also given a name, it's called the pupil. So let's just write that down somewhere over here. That hole is called the pupil. As you can see, the pupil is through which the light enters into our eyes. So, through the pupil light will enter into our eyes, and this is what enables us to see things. Now, our iris has a very important job. It's job is to control the size of this pupil, the hole. And that's because during the daytime, or when the ambient lighting conditions are very bright, then there are a lot of rays of light that are hitting our eyes. We wouldn't want too many rays of light entering our eyes, because, in that case, that might damage our cells. So, in such a case, what we do, is our iris will make its hole smaller. The pupil becomes constricted, so that it only allows the required amount of light to enter our eyes. On the other hand, if the ambient lighting conditions are very dark, so, let's say it's evening time, or night time, or you're inside a dark room, now there won't be many rays of light hitting your eyes in the first place. Now we would want to open up that pupil, otherwise you won't be able to see anything. And, so in such case, the pupil opens up, the hole opens up, allowing the light to enter. And you can do this experiment at your home. Just stand in front of a mirror in a dark room, your pupils will now be dilated, and just flash light into your eyes, and you will see immediately that pupil will get constricted and become smaller in size. All right, now let's get rid of the iris so that we can see the other parts of the eye. So, now we are only seeing a section of the iris. This is the same iris with the hole in between, all right? Let's look at the other parts. This bulge part of the eye, which is in front, is called the cornea. So, this is called the cornea. And over here we can see a lens, this is called as the lens of the eye. So, it's just called the lens. Let's write that down. This is called as the lens. And the space between the cornea and the lens is filled with a watery kind of a liquid, watery kind of liquid, and it's called the aqueous humour. It's called aqueous because it's watery. And, if you look at this carefully, you can now see that this cornea, along with the aqueous humour, and the lens, they're all convex shaped. The cornea and the lens together form a converging lens system. Whenever you want to concentrate on a specific object, their job is to make sure that the light from that object gets focused right at the back part of our eye, because it's the back part where we have lots and lots of light sensitive cells. So, the back part of the eye is completely covered with light sensitive cells, which we are seeing over here in red. This whole thing is also given a name, that's called the retina. Let's write that down, as well. So, let's write that down. This light sensitive cell covering, cell lining, is called retina. In order to see anything clearly, the light from that object must get focused exactly on the retina. If it doesn't, it'll look blurred to us. What do these cells do? Well, these cells, once light falls on them, they convert light into electricity and finally those electrical signals are carried from the retina all the way to our brain through some nerves. So, they'll be some nerves that carry all these signals. The nerves will connect to all the cells of the retina and all these electrical signals is carried to the brain. And this bundle of nerves are called optic nerves. Optic nerves. And then our brain, the command center, receives these electrical signals, does a lot of complex processing, and it's eventually able to figure out where those light rays came from, and then it constructs the image of the world around us. The whole thing is super complicated, our brain is super complicated, but it's also pretty amazing how it's able to do all of this. And, by the way, the space between the lens and the retina is filled with a jelly kind of transference substance called the vitreous humour. The word vitreous means looks like glass or kind of like glass. Of course it's not made of glass, it's made of organic substance, but just like glass it is pretty transparent. It's a jelly like transferring material and it's job is to maintain the shape of our eyes. Without this humour, our eyes could get easily crushed under the weight of the stuff that is on top of it. So the vitreous humour is a transferring, jelly-like substance. The aqueous humour is also transparent, but it's watery kind of substance. And the last thing we'll talk about are these fiber kind of things that we see over here, which are keeping our lens in place. They're called the ciliary, ciliary muscles. They have a pretty important job too. Their job is to be able to change the shape of this eye lens. Well, why do we need that? Well, we need that to change the power of our eyes, depending upon how far the objects are that we are looking at. This will make sense, if you draw some rays of light. So, let's dim all of these things, and consider some rays of light. All right, imagine we are looking at objects which are very far away. Then the rays of light from that object will be parallel to each other, and in order to see it clearly, those rays must get focused onto the retina. So, the ray diagram would look somewhat like this. Now, the important thing is the amount of bending that is required. Notice that these rays have to be bend by this much amount so as to get focused on the retina. And this bending is done by this lens system, converging system. Now, imagine that same object were to come closer. Again, to see it clearly, the rays of light must get focused onto the retina. So, these rays will not change, but, the incoming rays will now be diverging, and as a result, we will see now the required bending is higher, all right? Just concentrate on the bending when the object comes closer. Here it is. Can you see that? The required bending is more. More refraction is needed, meaning more optical power is needed. So, when objects are closer they require more optical power, more bending, and when the objects are farther away they require less optical power, less bending. Notice our eyes should be able to change its power depending upon how far the objects are, and that is accomplished by these ciliary muscles. When the object is far away, the required power is less, and so the required curvature of our lenses is also less. In such case, the ciliary muscles would be relaxed, like what's shown over here. But, when the objects come closer, since the required power is more, the ciliary muscles will start pushing on this lens to increase it's curvature as you can see, like this. I've exaggerated the figure over here. It'll push on the lens, increase the curvature, and as a result it will increase the optical power. And, that's how, depending upon the distance of the object from our eyes, the curvature of the lens will keep changing, and that is done by these ciliary muscles. And this phenomenon where ciliary muscles change the power of the eyes by changing the shape of the lens is called accomodation. So, that's their job, their job is to perform accommodation. And, by the way, we're going to look at accommodation in great detail in a future video, all right? So, don't worry too much about this accommodation as of now. That's pretty much it. Let's quickly summarize what we learned. We saw that the cornea, the aqueous humour, and the lens together form a converging system, whose sole job is to focus rays of light onto the retina. The retina contains light sensitive cells, whose job is to convert light into electricity. These electrical signals are carried out by optic nerves. There job is to carry the signal all the way to the brain, so that the brain can process the information. The jelly-like substance, the vitreous humour, maintains the shape of our eyeball. The iris regulates the size of the pupil, which in turn regulates the amount of light that enters our eyes, and, finally, the ciliary muscles regulate the power, optical power, of our eyes, depending upon the viewing distance