Human eye

The human eye is a sensory organ in the visual system that reacts to visible light allowing eyesight. It helps us see the world around us and do many things, from reading to playing sports. Besides seeing, the eyes also help keep our body’s daily rhythm and our balance.

The eye works like a living optical device. It is about the shape of a ball, with outer layers that keep light out except straight through the middle. Light first passes through the clear front part called the cornea—the clear part of the eye, which does most of the focusing. Then it goes through the pupil, the small opening in the colored part of the eye called the iris—the coloured part of the eye, which lets in just the right amount of light. After that, light goes through another lens inside the eye called the crystalline lens, which focuses the light onto a special layer at the back called the retina. The retina sends messages to the brain through the optic nerve so we can understand what we see.

Inside the retina are special cells that change light into signals the brain can use. Some cells, called rods, work best in dim light and help us see shapes and movement. Others, called cones, need brighter light and help us see colors and fine details. Together, these cells let us experience the world in full color and detail.

Structure

Humans have two eyes, one on each side of the face, sitting in bony areas of the skull called orbits. The front part of each eye includes a white outer layer, a colored circle called the iris, and a dark center called the pupil.

The eye isn’t a perfect ball; it has two main parts. The front part includes the clear curved surface called the cornea, the iris, and the lens. The back part contains a jelly-like substance and layers that help turn light into signals the brain can understand. Light enters through the cornea, passes through the pupil, and then goes through the lens to reach the back of the eye, where it creates the sense of sight.

Vision

See also: Visual acuity, Eye § Visual acuity, Fovea centralis § Angular size of foveal cones, and Color vision § Physiology of color perception

Field of view

The human eye allows us to see in different directions. When both eyes work together, we can see almost all around us. Each eye alone can see a little less, but together they give us a wide view.

Dynamic range

Our eyes can see very dim lights, like stars at night, and very bright lights, like the sun. This ability helps us see in many different lighting conditions.

The eye works like a camera, using a lens to focus light. The pupil acts like the camera’s opening, getting bigger or smaller to let in more or less light.

Movement

Main article: Eye movement

To see clearly while moving, our brains must turn our eyes to keep images steady. Humans have a special spot in the back of the eye called the fovea centralis that gives us sharp vision. The brain moves our eyes so that what we are looking at falls on this spot. Having two eyes helps us see in 3D, telling us how far away things are.

Our eyes move in many ways. During sleep, especially when we dream, our eyes move quickly. We also make fast jumps to look at different things, and our eyes can follow moving objects smoothly. When our head moves, our eyes move the opposite way to keep things clear. All these movements are controlled by muscles around the eye, helping us see the world clearly.

Main article: Rapid eye movement sleep

Main article: Saccade

Main article: Fixation (visual)

Main article: Vestibulo-ocular reflex

Main article: Pursuit movement

Main article: Optokinetic response

Near response

When we look at something close to us, our eyes need to adjust so we can see it clearly. This happens in three main ways.

First, our eyes move together toward each other to focus on nearby objects. This is called convergence. When we look at something far away, our eyes move apart a bit, which is called divergence.

Second, our pupils get smaller when we look at close objects. This helps make the picture clearer by reducing blurry edges and keeping more of what we see in focus.

Third, the lens inside our eyes changes shape to help focus light properly. This change in shape is called accommodation, and it lets us see things that are close to us more clearly.

Medicine

The human eye is very complex, so it needs special care from experts called eye care professionals. These professionals include ophthalmologists, optometrists, and others who help diagnose eye problems and prescribe glasses or contact lenses. Only ophthalmologists are trained to perform surgeries on the eye, and they can specialize in areas like the cornea, cataracts, laser treatments, retina, or oculoplastics. Other eye care professionals also play important roles in keeping our eyes healthy.

Pigmentation

Main article: Eye colour

Almost all mammals have brown or darkly-pigmented eyes. In humans, brown is the most common eye color, with about 79% of people in the world having it. Brown eyes happen because of a lot of a special material called melanin in part of the eye, which helps absorb light.

Amber eyes are a solid color with a strong yellow or gold look, often found in northern Europe and some other places. Hazel eyes can shift in color from brown to green and are common in parts of Europe. Green eyes are mostly found in Northern, Western, and Central Europe and happen because of a mix of brown and blue colors in the eye. Blue eyes are common in northern and eastern Europe and around the Baltic Sea. Gray eyes are similar to blue eyes but may have more of a material called collagen, which changes how light scatters.

Irritation

Eye irritation is when your eyes feel uncomfortable, like stinging, burning, or itching. People of all ages can experience this. Other signs include redness, dryness, excess tearing, and a feeling like something is in your eye. These symptoms can range from mild to severe.

Several things can cause eye irritation. For example, indoor air pollution, certain chemicals, and stress may play a role. How often you blink and how well your tears protect your eyes can also affect irritation. Eye care professionals, such as ophthalmologists and optometrists, help manage these issues. They might give you an eyeglass prescription to improve vision if needed.

Macular degeneration

Main article: Macular degeneration

Macular degeneration is common, especially in the U.S., affecting many people each year. It may be linked to lower levels of lutein and zeaxanthin in the eye. These nutrients act like a shield against damage from bright light, which can harm eye cells. Eating dark green vegetables like kale and spinach can help provide these important nutrients for eye health.

Sexuality

The human eye and the area around it, like the eyelids, eyelashes, and brows, have often been linked to beauty and attraction. Looking into someone's eyes, known as eye contact, is important in how people communicate without words. Features such as a dark ring around the iris (called a limbal ring), long eyelashes, and larger pupils can make someone appear more attractive. Historically, some people used natural methods to make their pupils appear larger to enhance their appearance.

Images

Here are some pictures showing different parts of the human eye. You can see the eye without labels, and also views with all the parts labeled so you can learn what they are called.