How Do Our Eyes Work?

Sight is something a lot of us take for granted, without much thought for how much detail is involved in the process. This article summarises just how our eyes function as well as what the common problems are when things go wrong.

The Eye As A Receptor

The human eye is a remarkable complex organ. It is more sophisticated than the eye in many other animals. Most simpler creatures can either only detect a light source with their eyes rather than being able to detect an actual image, or where images can be seem there is perhaps no colour perception. It is equally true to say, of course, that some creatures have far superior eyesight to humans, think of birds of prey for example.

The image that we perceive is in fact an interpretation of our brain to stimuli that are received through the eye. To understand this fully we should examine the eye in closer detail. The outer layer of the eye is called the sclera. This fully encloses the eye, but at the front (the part we can see) is transparent and here it is called the cornea.

Behind the cornea is the coloured part called the iris and in the centre of the iris is the pupil. The light gets into the eye through the pupil; this is always seen as black because no light escapes from inside of the eye. Contained within the sclera is a layers called the choroid; this is a layer containing many pigmented cells to make is very dark in colour. This is to prevent light bouncing around inside the eye and producing a confused image in the brain.

At the back of the eye, the inner most layer of the eye, is the retina. This is the part of the eye that converts light into electrical impulses that then travel to the brain. The retina contains two main types of cells: rods and cones. These are the cells that produce electrical impulses in sensory neurons which pass to the brain through the optic nerve that leads away from the eye.

Rod cells are the most effective in poor light, but they only produce a black and white image. Cone cells only work effectively in bright light. There are three types that react differently to different wavelengths of light:

• Red
• Green
• Blue

As a result of being able to distinguish these three colours in varying intensities we can see all of the colours of the spectrum. This is when they are working effectively of course, in some people they are not resulting in partial or complete colour blindness. To form an image on the retina light needs to be refracted. This occurs when light passes through one substance and into another, it essentially changes direction slightly.

Without this refraction we would not perceive images effectively, but because of it the image we look at is actually upside down when it reaches the retina. The brain then interprets this data and you essentially see the image you are looking at the right way up.


Perception Of Distance

The eye must be able to focus on objects at varying distances if it is to be an effective organ. If it could not do this then the majority of what we see would be out of focus (indeed it is for some people hence the need for spectacles). In order to achieve this, the lens of the eye must change shape in order to continue to focus on objects both close up and at a distance. The lens is an elastic material and can change shape easily and repeatedly (and importantly quickly as our eyes are always moving from object to object).

Two wellknown vision problems correctible via eyeglasses are nearsightedness and farsightedness. Nearsightedness focuses rays of light in front of the retina, while farsightedness focuses rays behind the retina– both of which result in blurred vision.

A concave lens can correct nearsightedness by bending incoming light rays outwards, so that the eye's lens (which usually bends incoming rays too much) focuses the light closer to the retina.
A convex lens similarly corrects farsightedness by bending light inwards slightly.

This article has been put together by the distance learning organisation Start Learning who are experts in home study. If you want to find out more about IGCSE Biology or many other distance learning courses please browse their website: Start Learning

A good way to find out more about IGCSE Biology is to sign up for a distance learning course on the subject. By studying in your free time and pace, you can gain the necessary knowledge while tailoring it to suit your schedule.

Kerrana McAvoy
Academic Director – Start Learning
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