 Let's explore the functions and the parts of the human eye. So if you were to look at the human eyeball, it's pretty spherical. And if you were to take a cross-section of it, this is what it would look like. There are a lot of parts, but let's look at one by one. The first part we're going to look at is the iris. The iris is what I've drawn in green over here. Most people have brown eyes. When we talk about the eye color, we're talking about the color of the iris actually. Some people have blue eyes and all of that. But what does iris really do? And what does it really look like? Well, it's kind of like a disk. This is a three-dimensional view. And you can see at the center of the iris, we have this dark thing. The dark thing that you were probably seeing, right? That's actually the hole in the iris, all right? And that's what we call the pupil. What's the job of the pupil? Why do you have a hole? Well, because we want the light to enter. So the light enters into the eye through that hole. But what's interesting is that the size of this pupil can change depending upon how much light we have. For example, if there is a very little light in this room, if it's very dark, then the pupil will actually become larger to allow for more light to come in. And if there's a lot of light, the pupil can actually become smaller. So you can see the pupil actually controls the amount of light that enters the eye. And let me show you that. So if you shine light into the eye, look, look, the pupil becomes smaller, just like what we saw over here. Okay, so going back, that's the iris and this hole in between is the pupil. What else do we have? Well, we have this bulged part which you can't really see because it's transparent. That's called the cornea. And its main job is to protect the sensitive parts of the eye so that if the damage happens to your eye, the first thing that gets scraped is your cornea, but it protects these things that are inside. And that brings us to the question, what are these things inside? What is this thing? Well, that's the main lens of the eye. Why do we want the lens? Because we want the light to be focused, right, on the back side of our eye and therefore we have a lens. And this lens is not the only thing that helps in focusing. You see, you have a liquid in between the lens and the cornea, which is like a watery liquid, which we call aqueous humor. It's called aqueous because it's a water-like substance. And if you now consider the cornea along with the aqueous humor, that also helps in focusing light. So both the cornea along with the aqueous humor and the eye lens, they together act like the converging lens system of our eyes. And their main job is to focus whatever light is coming onto the back part of the eye. But then what happens once you focus the light on the back part of the eye? Well, we want to send that information to the brain, but to do that, we need to convert light into electric signals. And that's why the whole back part is covered with cells that convert light into electricity. And this group of cells is what we call the retina. That's what retina do. Retina is basically a bunch of cells that convert light into electricity. So the retina is where the light is supposed to get focused. And then what happens once we have light getting converted into electricity? We need to send that electricity signals to the brain. So we need some kind of a wiring system. And that wiring system is what we call the optic nerves. It basically sends the electrical signals to the brain and then the brain processes that information. And now we might be wondering, well, what is there in this region? Is it like empty? It can't be empty because if it's empty, the eye will not be able to maintain its shape. It will be crushed very easily under the weight. So to maintain the shape, this is also filled with jelly-like substance, which we called the vitreous humor. It's vitreous stands for glassy. It's transparent. It needs to be transparent. That's why it's called vitreous humor. And its main job is to maintain the shape of the eye. OK, the final part that we'd be interested in are these tiny muscles over here, which we call the ciliary muscles. Why are they there? Their main job is to help in something called the accommodation. What does that mean? Let's go to the next slide to quickly understand what accommodation is. So imagine you have light incoming. You have imagine you have parallel light coming into your eyes. As we discussed, both the cornea and the lens together will focus the ray of light. In this diagram, I haven't drawn the converging from the cornea and the aqueous humor, but I have simplified it. But both of them are responsible. OK, remember that. OK, now to do that, we need to bend the rays of light. Isn't it? OK, but now imagine the object was closer. If the object is closer, then look, we need more bending power. We need to we need to have more power in this converging system. How do we get that? How do we adjust the power? How do we accommodate the power of our converging lens system? That's where the ciliary muscles come into the picture. So the ciliary muscles, when you need more power, they push on that. They contract and they push on this particular lens and they bulge it and increase its power. And similarly, when you need low power, they will relax. The ciliary muscles will relax and then they will this particular lens will become thinner and the power will decrease. This is basically what we call accommodation. And this is the reason why we have the ciliary muscles. We'll look at accommodation in more detail in the next video. But these are the main parts of the human eye.