The human eye is arguably the greatest sensory triumph evolution has ever produced. It is for the most part, an instrument that is finely tuned to our environment beginning from the moment we’re born. But what’s the magic behind it all? And what happens when the magic loses its spark?
Essentially, the eye is made up of parts which work in tandem to create the visual world that our brains process. The first three parts focus on adjusting the amount of light we perceive. The cornea is the clear film over our eye which focuses light into the eye; the iris – which is responsible for eye colour – adjusts the amount of light that is focused, and the pupil (the black centre of our eye), which decides how much light is physically let into our eye. The final stage of focusing is done by the lens, which directs all the light into the back of our eye – this precise focusing is done by the ciliary muscles, which control the shape of the lens.
After the light has been focused, it is directed to the retina. The retina is the fundamental part of the eye that transduces (i.e. converts) the light into neural signals, that can be sent to the optic nerve. The optic nerve feeds back into the visual cortex (located at the back of the brain) where electrical impulses register an image. While this may all seem relatively simple, the acuity of the entire system means that one small problem can have cascading effects on everything else.
The most common issue that can occur with the eye is myopia, commonly known as being short-sighted, which causes distant images to appear blurry. It is reported to affect up to 1/3 people in the UK, with it growing ever more common. Physiologically speaking, short-sightedness is thought to occur when the eyeball grows too long. Consequently, light cannot be focused properly onto the retina (instead focusing just “before” it), which creates a blurry image. Generally, short-sightedness is corrected using concave glasses, laser eye surgery or contacts.
Conversely, some people may suffer from hyperopia, or long-sightedness, which causes close-up images to appear blurry. This can be the result of an eyeball that is too short, a cornea that is too flat or more commonly: a hardened lens (also called presbyopia), which cannot adjust its focus properly. A hardened lens/short eyeball leads to light being focused “past” the retina, creating a blurry image. While this does occur in children and adolescents, it is more common as we get older, meaning that the longer we live the more likely it is we will all be forced to wear glasses regardless of how many carrots are in our diets.
Cornea: The clear first layer of the eye, which focuses light into the eye.
Lens: Transparent structure which changes shape in order to direct light to the retina.
Retina: The back layer of the eye, which converts light energy into electrical energy, to send to the optic nerve.
Optic nerve: A nerve which transports electrical energy from the retina to the primary visual cortex (also known as area V1) in the occipital lobe.
Myopia: Short-sighted vision, in which light focuses just “before” the retina, causing distant images to seem blurred.
Hyperopia: Long-sighted vision, in which light focuses just “after” the retina, causing close-up images to seem blurred.
Presbyopia: The loss of elasticity of the lens; the lens becomes hardened, meaning it cannot focus light into the eye as easily. This becomes more common with age.