How Does the Eye Work?

Have you ever wondered how your eye captures all the beautiful and colourful images around you? In this blog post, we explore the human eye and how this complex organ functions. From capturing light to transmitting images to the brain, we explain how the eye works step by step.

Structure of the Human Eye

The human eye, one of our most important sense organs, is a complex and finely tuned structure made up of multiple components that work together to capture and process visual information. Protected within the eye socket (orbit) is the human eyeball (bulbus oculi), composed of several layers. At the front of the eyeball lies the cornea, a clear and transparent layer that light first passes through.

Behind the cornea is the anterior chamber, filled with aqueous humour, which nourishes and protects the eye. Behind that is the iris, the coloured part of the eye surrounding the pupil, an opening that adjusts in size depending on light conditions. Located behind the iris and pupil is the eye’s lens, a flexible structure crucial for light refraction. Most of the eye’s interior is filled with the vitreous body, a gel-like substance that gives the eye its shape.

At the back of the eye lies the retina, a layer equipped with light-sensitive cells. The choroid, situated between the retina and the eye’s outer layer, supplies nutrients to the retina. The eye’s outer shell is formed by the sclera, a tough, white layer that provides protection and stability.

The conjunctiva is a thin, transparent layer that covers the white of the eye (sclera) and the inner eyelids. It acts as a protective barrier and keeps the eye moist with tear fluid. Finally, the optic nerve connects the eye to the brain and plays a critical role in transmitting visual information. Each part of the eye anatomy contributes uniquely to the structure of the eye and its ability to function.

How Does Vision Work?

Vision begins when light enters the eyeball through the cornea and is focused by the lens to create an image on the retina. The retina converts this image into electrical signals, which are sent via the optic nerve to the brain, where they are interpreted as visual perception.

• Light Entry: Vision starts as light rays enter the eye, first passing through the transparent cornea.
• Light Refraction: The cornea refracts (bends) light rays, directing them through the pupil, the central opening in the iris, which automatically adjusts to regulate the amount of light entering the eye.
• Focusing Through the Lens: Behind the pupil is the lens, which fine-tunes the light rays to focus them precisely on the retina.
• Image Formation on the Retina: The retina has millions of photoreceptors—rods and cones. Rods enable vision in low light, while cones are responsible for colour vision and sharpness.
• Signal Transmission to the Brain: The optic nerve transmits these electrical signals to the brain. Along the way, nerve fibres cross at the optic chiasm, integrating visual information from both eyes.
• Image Processing in the Brain: In the brain, particularly in the visual cortex (also called the cerebral cortex), these signals are interpreted, forming a conscious visual impression that enables us to perceive shapes, colours, movement, and depth in our surroundings.

The Functions of Each Eye Component

Each part of the human eye has a specific function that contributes to the complex process of vision.

Cornea

The cornea is the eye’s clear, transparent front surface, crucial for protecting the eye from dust, dirt, and foreign objects. It maintains eye structure and provides the initial stage of light refraction, essential for focusing light on the retina.

Iris and Pupil

The iris, the coloured part of the eye, surrounds the pupil and functions like a camera aperture, regulating light entry. The pupil adjusts in size to light conditions, narrowing in bright light and widening in darkness to protect the retina from excessive light exposure.

Lens

The lens, located behind the iris, fine-tunes light refraction and adjusts shape to focus on objects at various distances, a process known as accommodation.

Vitreous Body

The vitreous body is a gel-like substance filling most of the eye, supporting its shape and ensuring the retina stays in place, contributing to eye stability and clear light transmission.

Retina

The retina, a light-sensitive layer at the eye's back, contains rods and cones that convert light into electrical signals. This enables the perception of colours, shapes, and movement. With input from both eyes, it provides binocular vision for depth perception, allowing the brain to create a 3D view of surroundings.

Optic Nerve

The optic nerve transmits electrical signals from the retina to the brain. Comprising over a million nerve fibres, it’s essential for sending visual information to be interpreted as images.

Choroid

The choroid, situated between the retina and sclera, is rich in blood vessels, supplying nutrients and oxygen to the retina, crucial for retinal health and function.

Sclera

The sclera, or the white of the eyeball, is the eye’s outer protective layer, providing stability and protecting the eye’s sensitive internal parts from external influences.

How Do Vision Impairments Arise?

Vision impairments, such as nearsightedness and farsightedness, arise from deviations in the shape of the eyeball or the refractive power of eye components, affecting the eye’s ability to focus images accurately on the retina.

Nearsightedness (Myopia)

Nearsightedness occurs when the eyeball is too long or the cornea’s refractive power is too strong, causing light to focus in front of the retina. This results in blurry distant vision and often develops in childhood or adolescence.

Farsightedness (Hyperopia)

Farsightedness happens when the eyeball is too short or the cornea’s refractive power is too weak, focusing light behind the retina. This leads to blurry near vision, and in some cases, even distant objects may appear unclear.

Astigmatism

Astigmatism is caused by an irregular corneal shape, leading to light focusing on multiple points before or behind the retina, resulting in distorted or blurred images.

Presbyopia (Age-related Farsightedness)

Presbyopia is an age-related condition where the lens loses flexibility, making close-up vision more difficult. It generally begins around age 45 and can be corrected initially with reading glasses.

Common Eye Diseases and Their Effects on Vision

Although these parts of the eye are robust, they are susceptible to various eye diseases that can lead to visual disturbances.

Cataract: Cataracts develop due to the clouding of the eye's lens, which commonly occurs with age. This clouding leads to visual impairment, where vision may appear blurred or hazy. Colours may seem faded, and night vision can also be affected.

Macular Degeneration: Macular degeneration affects the macula, the part of the retina responsible for sharp, central vision. There are two main forms: dry and wet macular degeneration. This condition leads to a gradual loss of central vision, making it difficult to read, drive, and recognise faces.

Diabetic Retinopathy: In diabetics, there can be damage to the small blood vessels in the retina, known as diabetic retinopathy. This can result in bleeding, swelling, and impaired oxygen supply to the retina, which in turn affects vision.

Glaucoma: Glaucoma, often associated with increased intraocular pressure, gradually damages the optic nerve and can lead to a loss of peripheral vision. If left untreated, it can progress to blindness.