How the eye works

• Light rays enter the front of our eye through the clear cornea and lens. It is important that both the cornea and lens are clear. This allows the light to pass directly through the front of the eye to the back.
• The cornea and lens bend light so that it can focus on the retina at the back of our eye.
• The iris is the coloured part at the front of our eye, and the pupil is the black hole in the middle. The pupil changes size to allows different amounts of light into our eye.
• The pupil gets smaller in bright conditions to let less light in. The pupil gets bigger in dark conditions to let more light in.
• The middle of our eye is filled with a jelly-like substance gel called the vitreous. The vitreous is clear and allows light to pass directly from the front to the back of our eye.
• The retina is at the back of the eye. It is a light-sensitive layer which consists of light sensitive cells. There are two types of light sensitive cells: rod cells and cone cells.
• Rod cells are found around the edge of the retina. They help us to see things that aren't directly in front of us. This helps us to know what is around us. They help us with our mobility and getting around by stopping us from bumping into things. They also enable us to see things in dim light and to see movement.
• Cone cells are found in the centre of our retina called the macula. This is where the light is focused onto by the cornea and lens. Cone cells give us our detailed vision and colour vision. We use our detailed vision for things like reading, watching TV, looking at people's faces or screens.
• The retina collects the focused light and converts it into electrical signals, which are sent to the optic nerve.
• The optic nerve carries these signals to our brain. It is made up of around a million nerve fibres.
• The brain then processes the signals into the images we see.

Seeing can be likened to the process of taking photos on a traditional camera with a film. The retina is like a camera film which captures an image of what we are looking at. It is then sent to the brain where it is processed. This is like developing a camera film. Therefore, we actually "see" in our brain with the light information sent to it from our eyes. This whole process happens very quickly so that everything we see is in real time.

To see clearly, light needs to be focused sharply onto the back of the eye. A refractive error means that light is not focused correctly onto the back of the eye. This causes blurred vision.

There are four main types of refractive errors. These are:

• Myopia means short sighted. It causes your vision to be blurry in the distance but clearer when looking at things close up. It’s caused by the eyeball being too long or the cornea being more steeply curved, meaning light comes to a focus point before it reaches the retina. It is corrected by a minus lens prescription power.
• Hypermetropia means long-sighted. It means you find it more difficult to see things close to you clearly. It’s caused by the eyeball being too short. This means light doesn’t come to a focus point by the time it reaches the retina. It is corrected by a plus lens prescription power.
• Astigmatism is when the eye has irregular curves. These curves mean the light entering your eye does not focus at a single point on the retina. This can make vision blurry at all distances. Astigmatism is very common. Most people have some amount of astigmatism.
• Presbyopia is when as we get older, the lens in our eye becomes less flexible. This can make it harder for the eyes to focus for close up tasks like reading. This usually happens after the age of around 40. Presbyopia can be corrected with reading glasses.

Refractive errors can be corrected with glasses or contact lenses to help make things look clear again. However, glasses or contact lenses can only improve how light is focused in the eye, and can't improve vision which is affected by other eye conditions.