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Vision occurs when light enters the eye through the pupil. With help from other important structures in the eye, like the iris and cornea, the appropriate amount of light is directed towards the lens.Just like a lens in a camera sends a message to produce a film, the lens in the eye 'refracts' (in essence, bends) incoming light on to the retina. The retina, which is made up by millions of specialised cells known as rods and cones, transforms the image into electrical energy. This is sent to the optic disk on the retina where it is transferred via electrical impulses along the optic nerve to be processed by the brain.Some definitions and explanations of parts of the eye and how they function:

IRIS regulates the amount of light entering the eye and controls the widening and narrowing (dilation and constriction) of the pupil. It forms the coloured, visible part of the eye in front of the lens. 

PUPIL is the circular opening in the centre of the iris through which light passes into the lens of the eye. 

CORNEA is the transparent circular part of the front of the eyeball. It refracts the light entering the eye on to the lens, which then focuses it onto the retina. The cornea contains no blood vessels and is extremely sensitive to pain. 

LENS is a transparent structure situated behind the pupil of the eye enclosed in a thin transparent capsule and helps to refract incoming light to focus it onto the retina. 

CHOROID is the middle layer of the eye between the retina and the sclera. It also contains a pigment that absorbs excess light to prevent blurring of vision.

CILIARY BODY is the part of the eye that connects the choroid to the iris. 

RETINA is a light sensitive layer lining the interior of the eye.  It is composed of light sensitive cells known as rods and cones and works in much the same way as film in a camera. 

MACULA is a yellow spot on the retina at the back of the eye which surrounds the fovea. This is the area with the greatest concentration of cone cells. When the eye is directed at an object that part of the image focused on the fovea is the image most accurately registered by the brain. 

FOVEA forms a small indentation at the centre of the macula and is described as the area with the greatest concentration of cone cells. 

OPTIC DISK is the visible (when the eye is examined) portion of the optic nerve found on the retina of the eye. The optic disk identifies the start of the optic nerve where messages from cone and rod cells leave the eye via nerve fibres to the optic centre of the brain. This area is also known as the 'blind spot'. 

OPTIC NERVE leaves the eye at the optic disk and transfers all visual information to the brain. 

SCLERA is the white part of the eye, a tough covering which with the cornea forms the external protective coat of the eye. 

ROD CELLS are one of the two types of light-sensitive cells in the retina of the eye. There are about 125 million rods.  These also assist sight in dim light conditions. 

CONE CELLS are the second type of light sensitive cells in the eye's retina. The human retina contains 6-7 million cones; they function best in bright light and are essential for acute vision (receiving a sharp accurate image). It is thought that there are three types of cones, each sensitive to the wavelength of a different primary colour - red, green or blue. Other colours are seen as combinations of these primary colours.