How do the eyes work? Parts of the eye | Sightsavers (2024)

When we look at an object, light is reflected from it into our eyes, which enables us to see. Light enters 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.

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 thechoroid 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.

Our brain works together with our eyes to process the information we see and transform it into recognisable images. The clear disc-like part of the eye called the lens helps to focus light on the retina. The ciliary muscle adjusts the shape of the lens, helping it to focus on objects at different distances. This automatic focusing 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.

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.

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 astrachoma andriver blindness, all of which can cause blindness if left untreated.

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.

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