How does the human eye work?

Human eye

Stage 1: Light enters the front parts of the eye

Reflecting off objects in the world around us, rays of bright light continuously enter our eyes. Passing through the sclera (the white outer layer of the eyeball) and cornea (the transparent outermost layer, which covers each part of the eye), these rays are then transmitted into visuals that we can see.

These light waves are first refracted (bent) in such a way by the cornea upon passing through the round hole within the pupil (a black hole in the centre of the eye). Opening and closing like a camera shutter, the iris (a coloured, ring-shaped membrane behind the cornea) enlargers or decreases the size of the pupil to adjust the amount of light that passes through.

Stage 2: The lens adjusts focus

Continuing through the crystalline lens (a flexible and transparent structure positioned in the anterior portion of the eye), which acts like a camera lens, the light rays are refracted further and focused on the retina. This thin layer of tissue is like that of a film in a camera, and lines the back of the eye on the inside, near the optic nerve (a cable of fibre nerves that carry impulses from the retina to the brain).

Supplied with blood from a network of blood vessels and made up of millions of light-sensing nerve cells, your retina initially receives the light rays, after they have passed through the vitreous humour (a dense jelly-like substance that fills the globe of the eye) the wrong way round. These distinctively-shaped photoreceptor nerve cells forming a part of the retina are known as cones and rods.

Stage 3: Your brain correctly interprets the image

The cone cells are set in the centre of the retina, within the macula lutea or fovea - a section of the retina that's responsible for your central vision or visual acuity. Appearing as a yellow oval spot at the back of the eye, this area perceives high-resolution colours and fine details.

Found outside the macula and extending to the outer edge of the retina, the rod cells facilitate peripheral (side) vision. In low lighting conditions or at night, the rods help you to detect motion. Working together, the two different types of cells convert the light rays into electrical signals that pass through the optic nerve, which finally sends these electrical impulses to the brain, producing the image the correct way up.

People with vision problems, such as myopia (nearsightedness), hyperopia (farsightedness) or astigmatism, experience these refractive errors as a result of the image being incorrectly focused on the retina.

How contact lenses can help

Created to correct refractive errors, general vision problems and maintain eye health, contact lenses are optical devices which fit directly on the tear film layer of the cornea. Similarly to glasses, contact lenses refract and refocus light onto your retina, allowing you to see clearly at every distance. With the ability to control the focusing potential of your cornea, contact lenses are available in many different types, varying in design and wearing pattern.

Boxes of contact lenses

The most common type are soft contact lenses that offer high levels of comfort and flexibility. Soft contact lenses can correct eye conditions, such as myopia, hyperopia, astigmatism and presbyopia. Depending on your preferences and lifestyle, you can opt for daily contact lenses, monthly lenses, two-weekly lenses or even extended wear lenses.

Alternatively, gas permeable contact lenses or hard lenses are another option. These lenses are much more durable, can be worn for longer, and are especially helpful for wearers with astigmatism. Cosmetic lenses or coloured contact lenses are available in a range of colours and shades and can be used by those with nearsightedness, farsightedness or without any vision correction ('plano'). Before switching to contact lenses, or purchasing a new prescription, consult your optician for an eye test and a contact lens fitting.

Why are eye tests important?

Having regular eye tests isn't just useful for those with a change in prescription. An optician may spot early signs of eye conditions before you experience any symptoms - many of which can be treated if found early enough, such as diabetes or high blood pressure. This gives you a chance to review your treatment options and stop them from progressing.

Frequency of eye tests can vary from person to person but most optometrists advise having a thorough eye test at least once every two years. However, it is recommended that patients over 70 are tested more frequently (probably every 12 months) as they could be at higher risk of developing health conditions. It's also recommended that younger children are tested annually, as children's vision can change very rapidly at this stage of their lives.

A normal eye test