How The Eye Works

Light rays (from a light source or reflected off an object) enter our eyes through the clear cornea. The cornea bends or refracts the rays to pass through the pupil. The iris opens and closes the pupil to control the amount of light entering the eye. The light rays then pass through the lens, which changes its shape to finely focus the rays on the retina at the back of the eye. In the retina millions of tiny light-sensing nerve cells called rods and cones convert the light into electrical impulses. The optic nerve sends these impulses to the brain where an image is produced. The extraocular muscles control movement of the eyes and ensure that both eyes look at exactly the same object at the same time.

HOW THE EYE WORKS

The human eye is truly amazing. It focuses light to form images at the back of your eye, much like a camera. The eye instantly changes these images into electrical signals and sends them to your brain. The brain interprets the signals and you experience "seeing". The eye adapts to a wide range of lighting conditions and focuses almost instantly over a tremendous range of distances.

STRUCTURE AND FUNCTION

FRONT OF THE EYE

Structures in the front of the eye are responsible for focusing and controlling the amount of light let into the eye.

Cornea: The cornea is the clear dome on the front of the eye. It, not the lens, does most of the eye's focusing. It focuses light by bending it so the light rays form an image on the retina. Since the cornea has the greatest bending (focusing) power, it is the cornea's shape that determines a great deal of quality of your vision.

Iris & Pupil: The iris is the coloured part of the eye. It acts much like the iris of a camera, opening and closing, to regulate the amount of light entering through the pupil (the dark opening in the centre the iris).

Crystalline lens: The lens is located behind the iris. It fine tunes focus by further bending light rays as they pass through the eye to form an image on the retina. The lens must be clear. Cloudiness of the lens is referred to as a cataract.

BACK OF THE EYE

The eyeball (or globe) is comprised of three layers.

Sclera: The sclera is the fibrous white outer layer that gives the eyeball it's shape.

Choroid: The choroid is a vascular layer that lies inside the choroid and underneath the retina.

Retina: The retina is the inner most layer lining the inside of the back of the eye. Like the film in a camera it is thin and delicate and captures information from light that strikes it to create an image. Millions of tiny light-sensing nerve cells (photoreceptors) collect information from the light and send electrical signals to your brain via the optic nerve. Your brain interprets those signals to create what you experience as images. As with the film in a camera, damage to the retina creates irreparable damage to the image that cannot be corrected by focusing more (i.e. 'stronger' glasses).

There are two kinds of photoreceptors; rods and cones. Cones are present throughout the retina, but are especially concentrated in the centre in an area called the macula, and provide clear, sharp central vision and detect colours and fine details. Rods are located outside the macula extending all the way to the periphery. They provide peripheral vision, detect motion, and help us see in dim light.

Macula and fovea: The macula is a very specialised highly pigmented area near the centre of the retina. It has a diameter of around 6 mm. Near its centre is the fovea, a small pit that contains the largest concentration of cone cells in the eye and is responsible for central, high resolution vision.

ON THE OUTSIDE

Optic Nerve: The optic nerve is a big cable consisting of hundreds of thousands of nerve fibres that carry the signals from the photoreceptors in the retina to the visual centres in the brain. Damaging the nerve is akin to cutting the cable between a security camera and the monitor or your DVD player and the TV. No cable equals no image no matter how advanced the rest of the equipment is. The optic nerve is an extension of the brain and part of the central nervous system. As mammalian central nervous system fibres are incapable of regeneration, optic nerve damage produces irreversible blindness.

Extraocular muscles: On the outside of each eye are six muscles (extraocular muscles) that attach to the sclera and control the movement and alignment of the eyeballs. Malalignment results in double vision.

HOW WE SEE