Eyes and Vision

How the Eye Works

The human eye is a roughly spherical organ, about 2.4 centimetres in diameter, that converts light into electrical signals the brain can interpret as images. It does this with extraordinary precision — capable of distinguishing millions of colours and detecting a single candle flame from about 1.6 kilometres away in darkness.

Vision is often compared to a camera, and the analogy is quite useful: the eye has a lens that focuses incoming light, an adjustable aperture (the pupil) that controls how much light enters, and a light-sensitive surface at the back (the retina) that captures the image.

To understand how the brain processes the signals the eye sends, visit our brain anatomy guide.

Key Structures of the Eye

Light enters the eye through the cornea — the transparent, dome-shaped front surface that performs most of the eye's focusing work. Behind the cornea is a fluid-filled space containing aqueous humour, which nourishes the cornea and maintains eye pressure.

The iris is the coloured ring of muscle that surrounds the pupil — the dark circular opening in the centre. The iris expands or contracts to control how much light passes through, like a camera's aperture. In bright light the pupil shrinks; in dim light it widens, sometimes to about 8 millimetres.

Behind the iris sits the lens — a flexible, transparent disc that fine-tunes focus. Tiny ciliary muscles surrounding the lens change its shape to bring objects at different distances into sharp focus, a process called accommodation.

The large central cavity of the eye is filled with a gel-like substance called vitreous humour, which maintains the eyeball's shape and holds the retina in place.

The Retina: Light Detector

The retina lines the back of the eyeball like the film in a camera. It is only about 0.5 millimetres thick but contains over 120 million specialised light-sensitive cells called photoreceptors.

There are two types. Rods number about 120 million and are scattered across the retina (except at the very centre). They are extremely sensitive to low levels of light and detect movement and shapes in dim conditions, but they do not distinguish colours.

Cones number about 6 to 7 million and are concentrated in the fovea, a tiny pit at the centre of the retina. They require more light to function but provide colour vision and the sharpest detail. There are three types of cone, each sensitive to different wavelengths — roughly corresponding to red, green and blue light. The brain combines signals from all three to produce the full spectrum of perceived colours.

From Eye to Brain

When photoreceptors detect light, they generate electrical signals. These pass through several layers of retinal cells — including bipolar cells and ganglion cells — and the signals are already partially processed before they leave the eye.

The axons of ganglion cells bundle together to form the optic nerve. The two optic nerves (one from each eye) meet at the optic chiasm, where signals from the left half of each retina cross to the right side of the brain, and vice versa. This means the left visual cortex processes the right side of your visual field, and the right cortex handles the left side.

The signals travel to the lateral geniculate nucleus in the thalamus, then onward to the primary visual cortex (V1) at the very back of the brain. From there, visual processing extends into specialised areas that handle colour, motion, depth and object recognition. Remarkably, about 30% of the brain's cortex is involved in some aspect of vision.

Common Vision Differences

Many people have refractive differences — conditions in which the shape of the eye means light does not focus perfectly on the retina.

Myopia (short-sightedness) occurs when the eyeball is slightly too long or the cornea is too curved, so distant images focus in front of the retina instead of on it. Close objects are clear; distant ones are blurry. It is extremely common and usually corrected with concave (minus) lenses or contact lenses.

Hyperopia (long-sightedness) is the reverse: the eyeball is too short, so close objects are harder to focus. Convex (plus) lenses correct it.

Astigmatism occurs when the cornea is not perfectly spherical but more oval-shaped, causing blurring at all distances. It is also very common and correctable.

Presbyopia is an age-related loss of the lens's flexibility, making it harder to focus on close objects. It typically begins in the mid-forties and is the reason reading glasses become common with age.

Colour Vision

Colour vision depends on the three types of cone in the retina — labelled S (short wavelength, responding most to blue), M (medium, green) and L (long, red). The brain compares the signals from all three to work out colour.

Most people described as "colour blind" are not unable to see colour at all — they have reduced ability to distinguish certain colours, most commonly red and green. This results from having an altered or missing cone type. Red-green colour vision deficiency is much more common in males (about 8%) than females (about 0.5%), because the genes for M and L cones are on the X chromosome.

Depth Perception and Two Eyes

Having two forward-facing eyes gives humans excellent depth perception — the ability to judge distances accurately. Each eye sees the world from a slightly different angle, so the two retinal images are not identical. The brain compares the small differences between them (called binocular disparity) to calculate how far away an object is.

This depth cue, called stereopsis, is most effective for objects within a few metres. For greater distances, the brain relies on other cues such as relative size (familiar objects appear smaller when farther away), perspective (parallel lines seem to converge in the distance) and motion parallax (nearby objects move across the visual field faster than distant ones when your head moves).

People who have vision in only one eye lose stereopsis but can still judge depth effectively using these monocular cues — a remarkable demonstration of how adaptable visual processing is.

Protecting Your Eyes

The eyes are sensitive organs, but many common threats to vision are preventable. Strong ultraviolet (UV) light can damage the retina and accelerate cataract formation over time. Wearing sunglasses that block both UVA and UVB rays is a simple protective measure in bright conditions.

For those who spend many hours on screens, digital eye strain — characterised by dryness, fatigue and temporary blurring — is common but not damaging. Taking regular breaks, blinking consciously and adjusting screen brightness can reduce discomfort.

Regular eye examinations are important for detecting changes in vision and identifying conditions such as glaucoma early. This is educational context only — for personal eye health advice, consult an optometrist or ophthalmologist.

You can explore more about the senses with our companion guides: ear anatomy and smell and taste. Use the body systems explorer to see how vision connects to the nervous system, and test your knowledge with the anatomy quiz.