Your eyes and how they work

The eye is the organ of sight and is a direct extension of the brain. It's an amazing structure, an inch in diameter, and roughly the size of a ping pong ball. It sits in the eye sockets of the skull. The eyelids keep the front part of the eye moist and clean by blinking.

The human eye is made up of three layers. The tough outer layer, also known as the 'white of the eye' is called the sclera and it's main function is to protect and maintain the shape of the eyeball. The sclera covers most of the eyeball and the muscles that move the eye are attached to it.

The sclera is continuous with the transparent dome shaped structure at the front of the eye, known as the cornea. This acts as the 'window of the eye' and helps to focus the light rays that enter the eye.

Behind the cornea, is the coloured part of the eye called the iris.  It has muscles attached to it, which help it to change shape and control the amount of light that enters the back of the eye through a black circular opening in the centre of the iris called the pupil.  The pupil gets larger in dim light and smaller in bright light.

Between the iris and cornea, is a space called the anterior chamber. The anterior chamber is filled with a special fluid called aqueous humour which helps to nourish the eye and keep it healthy.

Behind the iris, sits the lens. The lens is clear and colourless and its role is to focus light rays entering the eye through the pupil onto the back of the eye. The lens is suspended in the eye by a bunch of fibres attached to a muscle called the ciliary muscle. The ciliary muscle is able to change the shape of the lens; it makes the lens thicker when looking at objects up close and thinner when looking at distant targets.

The largest part of the eye lies behind the lens and is a space called the vitreous chamber. It forms two thirds of the eye's volume and gives the eye it's spherical shape. The vitreous chamber is filled with a clear, jelly like substance known as the vitreous. Light passing through the lens shines straight through the vitreous to fall on the back of the eye.

The back of the eye is called the retina.The function of the retina is to take the light information that enters the eye and to change it into nerve signals so that the brain can understand what the eye is seeing. The retina holds millions of light processing cells called rods and cones. Cones are responsible for colour vision while rods allow us to see the form or shape that an object has. Rods are also responsible for vision in dark conditions.

From the retina, this information is then passed to the brain through a bundle of nerve fibres called the optic nerve.

How The Eye Works

The human eye works in a similar way to a camera. Light rays reflected off the objects that we see enter the eye initially through the cornea which behaves very much like a lens cover. The cornea takes the light rays and bends them through the pupil, which we know as the dark round opening in the centre of the coloured iris. The iris and pupil represent the aperture of a camera.

Just as an image is projected onto a film at the back of a camera, light rays that have entered the eye are focused on the back layer of the eye, the retina.  The image formed on the retina is upside down. These light rays are then changed into electrical signals and travel via the optic brain to the brain for further image processing.

Dr Helena Hurairah