How your eyes work
Vision involves two important organs in your body – your eye and your brain. Light enters the front of your eye, is turned into electronic signals at the back of your eye and those signals are interpreted by your brain so you can ‘see’.
The way the eye works has been compared to a digital camera. The cornea, the clear front surface, acts like a camera lens to begin focusing the light. The iris, the coloured part of the eye, opens and closes like the diaphragm of a camera. This changes the amount of light that gets in through the pupil.
The crystalline lens is directly behind the pupil and acts like your camera’s autofocus function, adjusting so you can focus on things that are close to you or further away. If the lens becomes cloudy, we call it a cataract. Cataracts can be removed and replaced with an artificial lens.
After passing through the cornea and the lens, light reaches the retina. This light-sensitive lining at the back of your eye is like the digital camera’s sensor. It has about 130 million cells called rods and cones that convert optical images into electronic signals. These signals are then transmitted to your brain via the optic nerve.
The eye also has other supporting structures including muscles that allow it to move, ducts to carry fluids like tears and blood to lubricate or nourish the eye, and nerves that send sensory information, such as pressure or pain, to the brain.
Parts of the eye
Iris: the coloured, visible part of your eye that regulates the amount of light that enters. The iris controls the widening and narrowing of the pupil.
Pupil: the circular opening in the centre of the iris. Light passes through the pupil into the lens of your eye.
Cornea: the transparent circular part of the front of the eyeball. It refracts the light entering your eye onto the lens, which then focuses it onto the retina. The cornea contains no blood vessels and is extremely sensitive to pain.
Lens: a transparent structure that sits behind your pupil and focuses light onto the retina.
Retina: a layer that lines the interior of your eye, composed of light sensitive cells known as rods and cones. Rods are necessary for seeing in dim light while cones function best in bright light. There are about 125 million rods and between 6 and 7 million cones in the human eye.
Sclera: the white part of your eye. This is your eye’s external protective coating.
Choroid: the middle layer of your eye between the retina and the sclera. It also contains a pigment that absorbs excess light so preventing blurring of vision.
Ciliary body: the part of the eye that connects the choroid to the iris.
Fovea: This is small indentation at the back of your eye with the highest concentration of cone cells. When you look at something, the part of the image focused on the fovea is what you see most clearly – your ‘central vision’.
Macula: the area surrounding the fovea.
Optic disc: Where the optic nerve enters your eye it creates a ‘blind spot’ on the retina. This spot, the end of the optic nerve, is also called the optic disc and is visible when your eye is examined.
Optic nerve: This nerve leaves your eye at the optic disc and transfers all the visual information to the brain.