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How do the eyes work?

Our eyes are responsible for
80 per cent of all the information our brain receives. Here you can find out more about how we see.

Kalpana gets her eyes tested in India. © Sightsavers/Atul Loke

How do we see?

The images we see are made up of light reflected from the objects we look at. This light enters the eye through the cornea, which acts like a window at the front of the eye. The amount of light entering the eye is controlled by the pupil, which is surrounded by the iris – the coloured part of the eye. The pupil changes size according to how much light is present; it is smaller in bright light and becomes larger when there is less light.

As the cornea is curved, it bends the light entering the eye, creating an upside-down image on the retina. The retina is a complex part of the eye, and its job is to turn light into signals about images that the brain can understand. Only the very back of the retina is light sensitive: this part is roughly the size of a 10p coin. It is packed with photosensitive cells called cones and rods, which help us to see in the daytime and at night.

A diagram showing the parts of the eye, including the cornea, lens, iris and ciliary muscle at the front of the eye, plus the aqueous humour. At the rear, it shows the sclera, choroid, retina and optic nerve. The vitreous humour is in the centre.

Other parts of the eye include the aqueous humour, which is a liquid that sits in a chamber behind the cornea. It keeps the eye nourished and helps it maintain an optimum pressure so that the eyeball remains spherical. The vitreous humour is a clear gel that fills the space between the lens and the retina. It also keeps the eye healthy and maintains its round shape.

The sclera is the white part of the eye, forming an outer layer that protects everything inside, while the choroid is the layer of the eye that lies between the retina and the sclera. It is made up of layers of blood vessels that nourish the back of the eye.

How do we see images?

The lens is a clear disc-like part of the eye that helps to focus light on the retina. It can do this because it is adjustable, and uses a muscle called the ciliary muscle to change shape and help us focus on objects at different distances. The automatic focusing of the lens is a reflex response and is not controlled by the brain.

Once the image is clearly focused on the sensitive part of the retina, energy in the light that makes up that image creates an electrical signal. Nerve impulses can then carry information about that image to the brain through the optic nerve, which is a collection of more than a million nerve fibres. As the cornea bends light when it enters the eye, the brain receives images that are upside down so it turns them the right way up when it processes the information.

How do we see in colour?

Cone cells located on the retina at the back of the eye are responsible for daylight vision. They enable us to see images in colour and detail. There are three types of cones, each sensitive to a different wavelength of light: red (long wavelengths), green (medium wavelengths) and blue (short wavelengths).

We can see colours other than red, green and blue because the cones can detect additional wavelengths of light and work together to produce different colours. The brain is able to interpret the signals sent from the cones into colours. It is thought that the human eye can perceive around a million colours, although people whose eyes have a fourth cone can see even more.

Rods, found alongside the cones, are responsible for night vision. They are sensitive to light but not to colour, meaning we can only see shades of grey in low-light conditions. In darkness, the cones do not function at all. Animals that are active at night can see in the dark because their eyes contain millions of additional rods.

217 million
people have an untreated severe visual impairment
800 million
people have unaddressed refractive error
1.8 billion
people have an age-related near-vision impairment

Common eye conditions

Refractive errors are eye disorders caused by irregularity in the shape of the eye. This makes it difficult for the eyes to focus images clearly, and vision can become blurred and impaired.

Short sight (myopia) and long sight (hypermetropia) are common conditions, both caused by the cornea and lens not focusing properly on the retina. Short sight is where the eyeball is elongated or the lens is too thick, causing the image to focus in front of the retina. Long sight is where the eyeball is too short or the lens too thin, causing the image to focus behind the retina. Prescription glasses can help with both long and short sightedness.

Cataracts are the leading cause of blindness. In this eye condition, the lens becomes cloudy or less transparent, blocking light from passing through to the retina and resulting in blurred vision that can progress to blindness. There are other sight-threatening eye conditions and eye diseases that are not so easily corrected, including glaucoma and neglected tropical diseases such as trachoma and river blindness, all of which can cause blindness if left untreated.

What we’re doing to protect sight

For more than 70 years, Sightsavers has been working with partners to prevent or treat these and other causes of avoidable blindness that affect hundreds of millions of people in low and middle income countries. We do this not only by distributing treatment and carrying out operations to restore sight, but also by improving health care in local communities to enable more people to have their eyes checked, and to ensure they can be treated if they need it.

You can find out more about what we do, or support Sightsavers’ work and help people to see again by making a donation.

A close-up of Patience wearing glasses and a face mask.
Patience from Liberia received glasses to correct refractive error.
© Sightsavers/Carielle Doe

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More about the eyes

A close-up of a patient's face showing the milky cataracts in their eyes.

What are cataracts?

Cataracts are an eye condition that causes blurred vision, and can lead to blindness. Our goal is to make sure surgery and treatment is available for those who need it most.

Aluna from Tanzania has her eyes checked for trachoma. They are visibly red and swollen.

What is trachoma?

Trachoma, a neglected tropical disease, is the world’s leading infectious cause of blindness. Repeated infections cause the eyelashes to turn inwards.

A woman wearing glasses holds one hand over her right eye during an eye test.

What is refractive error?

Refractive errors are irregularities in the shape of the eye, causing blurred vision. We aim to improve access to global healthcare so this can be treated.

Find out about other eye conditions we treat

Protecting sight