Eyes – how your eyes work

      When you look at an object, the light from it enters your eye through the pupil. The iris changes the size of the pupil, depending on how bright the light is. The lens focuses the light onto the back of the eye: the retina. The retina is a mass of light-sensitive neurons, called photoreceptors, which change light signals into electrical ones.

Contents

• How a camera works

• How your eye works

• Inside your eye

Your eyes are like a wonderful kind of camera.
They take pictures of the world around you and send the pictures to your brain. Your brain works out what your eyes are seeing. This happens from the moment that you open your eyes in the morning to when you close your eyes at night.

How a camera works

          The light rays from an object pass through the lens of the camera and get recorded on a film or a computer chip.
Do you notice something about this drawing? Yes, the picture that is recorded by the camera is upside down (of course, when you look at the picture as a printed photo or on a computer screen, it is not upside down.)

How your eyes work

          Your eye works in a similar way to a camera - light passes through the lens of your eye and is 'recorded' on the back of your eye (the retina).

Do you notice something about this drawing? Yes, the picture that your eye takes is upside down too!

          Why don't you see things upside down?

          Well, your eye sends the picture to your brain, and your brain turns the picture the right way up and tells you what you are looking at. So you see things the right way up.

Inside your eye

What are the parts of your eye and what do they do?

Cornea
          This is the see-through skin that covers the front of your eye. It is clear like glass and it has no blood vessels in it.

Sclera
          This is the tough skin which covers the outside of the eyeball (except for the see-through cornea). We call it the 'white' of the eye.

Iris
          The iris controls the amount of light that enters the eye. The iris is the coloured part of your eye.

 Pupil
          This is the hole in the coloured iris. It lets light into your eye.  It gets very small in bright light, and bigger in dull light.

The lens
          The lens focuses light onto the retina. It changes shape to make sure that the 'picture' on the retina is as clear as possible.

Retina
          This is like a movie screen which shows the picture you are seeing - upside down, remember? The retina has two lots of cells called 'rods' and 'cones' (because that is what they look like.) Rods can 'see' black and white.  Cones can 'see' colours. They turn the picture into an electrical message for the brain.

Blind spot
          This is a bit of your retina which is not sensitive to light because there are no rods or cones there. It is the spot where the optic nerve is joined on to the retina.

Optic nerve
          The electrical messages from the retina travel along the optic nerve to your brain. It's a bit like the cable that carries all the TV pictures from your aerial to your TV so that you can see the programs. The great thing is that our eyes take these 'movies' all the time and we don't need any film or camera.

Eyelids and Eyelashes
          These protect your eyes. The eyelids can shut out light so that you can sleep. They will shut very fast if they feel something that is trying to get into your eye.

Eyelashes
          Eyelashes are very sensitive, and if they feel dust coming they trap it as your eyelids close.

Eyebrows
          Eyebrows also help to keep dust and sweat out of your eyes.

Tear glands
          These are small glands inside your upper eye lid. Their job is to make tears to keep the surface of your eyeball clean and moist, and help protect your eye from damage.
When you blink, your eyelids spread the tears over the surface of the eye. Small things that are on your eye (like specks of dust) wash into the corner of your eye next to your nose. Sometimes tears flow over your lower eyelid (when you cry, or you have hay fever), but mostly the tears flow down a tiny tube at the edge of your lower eyelid, next to your nose. (If you look very carefully you can see a tiny dot that is the beginning of that tube). This tube carries the tears to the back of your nose (and this is why your nose 'runs' when you cry!)

Conjunctiva
          This is the lining on the inside of your eyelid and the outside of the front of your eye (except for the special skin of the cornea). You can see some tiny blood vessels on the conjunctiva over your eye. If your eyes get sore, these blood vessels get bigger and your eye looks red.

There are two lots of fluid in the eye.

Aqueous humour
          Aqueous means water, and humour means fluid.  This watery stuff fills the front of the eyeball around the lens.

Vitreous humour 
          This is a thicker jelly-like liquid which fills the larger part of the eyeball and keeps it in shape. (Vitreous means glassy, because the vitreous humour is very clear, so that light can pass through it).

Ciliary muscles
          These are a circle of tiny muscles around the lens. They change the shape of the lens by squeezing and relaxing. They squeeze (making the lens fat) to look at nearby objects, and relax (making the lens thinner) for far away objects.
Your eyes are very beautiful and also very clever, because all the different parts work together to help you see!
And that’s how you see: Light, reflected from an object, enters the eye, gets focused, is converted into electrochemical signals, delivered to the brain, and is interpreted, or "seen," as an image.