In the human eye, light enters the pupil after being focused by the cornea, the clear window at the front of the eye. The image is then accurately focused on the retina by the lens. Light-sensitive nerve cells called rods (for brightness) and cones (for colour) react to the light. They interact with each other and send messages to the brain via the optic nerve that indicate brightness, colour, and contour. The eye is roughly spherical, filled with a transparent gel-like substance called vitreous humour.
The cornea is the transparent outer 'window' of the eye, which covers the iris and the pupil at the front of the eye. The cornea, together with the lens, refracts or changes the direction of light to focus it on the retina.
The iris is the coloured part of the eye which surrounds the pupil. It controls the amount of light going into the eye by changing the size of the pupil using tiny muscles to contract and dilate it.
The pupil is the variable-sized, black circular shaped opening in the centre of the iris that regulates the amount of light that enters the eye. It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye. When it is very light the pupil is small and when it is dark the pupil gets bigger to allow more light into the eye.
The lens is a transparent part of the eye behind the iris. The purpose of the lens is to focus light onto the retina at the back of the eye by bending light rays so that they form a clear image on the retina. The lens is elastic, so it is able to change shape to focus on to the retina. Its shape gets fatter to focus close objects and thinner to focus distant objects on the retina.
The aqueous humour is a water-like fluid which fills the front of the eye between the lens and cornea and provides the cornea and lens with oxygen and nutrients.
The retina is a thin layer of nerve cells which contains millions of photoreceptors that form the light-sensitive inner lining at the back of the eye. Rays of light enter the eye and are focused on the retina by the cornea and lens. The retina converts light into electrical impulses to produce an image which is sent along the optic nerve to the brain to interpret. There are two types of photoreceptors: rods work in low light and are responsible for night vision and cones work best in bright light and provide perceptions of colour and fine detail.
The macula is the small sensitive area at the centre of the retina about the size of a pinhead which is responsible for central vision, i.e. what we see straight in front of us. The macula is a very specialised part of the retina which enables the vision needed for detailed activities such as reading and writing, and to recognise colours.
The fovea is the central point of the macula and gives the sharpest vision.
The sclera is the white of the eye and forms the tough, opaque, outer coating of the eye. Six tiny muscles are connected to it which control the eye’s movements.
The optic nerve is a bundle of more than one million nerve fibres that carries visual signals representing colour, light and dark from the retina to the brain which then interprets what we see. Where the optic nerve leaves the retina there are no sensory receptor cells, so there is effectively a blind spot.
The vitreous humour is the clear, jelly-like substance that fills the inside of the eye from the lens to the retina.
The conjunctiva is a thin, clear membrane covering the front of the eye and inner eyelids which forms a first layer of protection against infection. Cells in this lining produce mucus that helps to lubricate the eye.