How do my eyes see?

How do eyes work?

It’s always nice to have an understanding of what is going on in the eyes and we do get asked how do the eyes see? Now we'll look at this purely and simply from the anatomy of the eyeball itself, how each part of the eye helps the eye to see - how we get the light focused onto the back of the eye. The strange thing about all this is, that you actually see with your brain, your eye catches the light and sends that message off to your brain and your brain pieces it all together and makes the picture. Still with me? Okay. So we'll start off at the beginning. We'll work through it as if we are following the light going into the eye.

So first of all, we have the very small, transparent window right at the front of the eye. That is called the cornea and you can see the pupil and the iris through the cornea, so it has to be transparent, otherwise the light can't get in.  The cornea is curved so that it will bend the light going into the eye. That's the first surface that's going to bend the light going into the eye, trying to get the light focused into the back of the eye. So it'll get a nice sharp image. Now the next thing to talk about then is the iris, which is the coloured part of your eye around the pupil, which is that little black hole in the iris. Now the pupil is literally just a hole. Yes, it's just a hole in the iris.

The iris is a muscle that gets bigger and smaller and hence your pupil gets bigger and smaller in certain light conditions. The reason that the pupil is black, is because no light comes out of your eye. Things only have a colour because a certain wavelength of light comes off those things. Now the reason we get red eye in some photographs, if there's a really bright flash, is because the back of the eye, the inside of the eye itself is actually red. And if you throw so much light into the eye, you can get a reflection off the back of the eye and you get a red pupil. So you're actually seeing the back of your eye when you see red-eye. So the light will enter the eye through the pupil.

Now, as I said, the iris is just a muscle that gets bigger and smaller depending on the light. So if it's very dark and there's not enough light around, the iris will get bigger and will move out the way. And therefore your pupil gets bigger to let more light into the eye. If it's very, very bright, what will happen is your pupil will shut down. It'll get smaller so that the iris muscle will actually contract to make that smaller. So we get less light in the eye if there's too much. So the next thing that happens is behind the pupil, so inside your eye behind the pupil is the lens. Now the lens is very clever because the lens is there to try to help us to see things at different distances. So the lens will change its shape to try to focus that light from coming at us from different distances onto the back of the eye, because it’s all about getting that light from the front of the eye into the back of the eye onto the retina.

So as we move through the eye, very back of your eye's called the retina. This is the bit where you get all the photoreceptor cells. But what is a photo receptor? It's basically a little cell that's going to catch the light. It's going to change that light into an electrical impulse. And so the retina is made up of these photo receptor cells and nerves, essentially. So it's going to catch the light in these photo receptors. It's going to change that into an electrical impulse and send it off down the nerves and down your optic nerve and into your brain. It's going to come all the way around your brain and it's actually the very back of your head that works it all out (the occipital lobe).

This is the visual cortex. That's what you see with. Now, we want that light to get to the retina. We want that light to get nicely onto the retina at the very back of your eye. We need a nice sharp image, a nice sharp focus. So all of the different curved parts in the eye, like the cornea, then the lens, they are all trying to get that sharp focus onto the retina. And that will give you a nice sharp image and therefore you'll have good vision. If the image falling on the retina, isn't sharp. For example, if you're short-sighted or if you're long-sighted, or if you have astigmatism, the image isn't quite getting onto the retina.

That's when your vision will be blurred. So, that's very basically how we see with our eyes. As I say, we don't actually see with our eyes, they catch the light. It's a very clever tool, catching the light, converting that light into an electrical message that our brain then works out and pieces it all together. And we can see. It's amazing really.

So that's the different parts of the eye, how the light passes through the eye, how they all work together. It's very important that they all work together to get that focus to where it needs to be.


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