For most people, it’s a matter of course: You open your eyes and you see the world. When the alarm clock goes off, you see the red numbers, the colour of the bedspread and your pyjamas, the green leaves on the tree outside the window and the ceiling, which could probably use another coat of paint. Vision supplies us with 80% of all the impressions from our surroundings.
The average human eye has a diameter of 24 millimetres and weighs just about 7.5 grams. It is composed of 6 grams of water and 1.5 grams of cell tissue. Low materials cost for such a powerful and lively camera. It has not yet been explained down to the last detail exactly how the two marble-shaped organs manage to project so perfect an image of the world around us into our consciousness.
The basic functions have been explained. Like a camera, the eyes have their very own optical system. The most important components of the eye are:
Regardless of whether you’re looking at the latest fashion magazine or watching an exciting football game on television – the light rays reflecting off the magazine or the television screen penetrate our eyes. The first thing they strike is the cornea. It is visible and palpable from the outside and has to deal with dust off the street as well as toxic fumes.
After the cornea, the light rays traverse the anterior eye chamber and the pupil in the iris. The Germans named the iris “Regenbogenhaut” (rainbow membrane) for its beautiful colour. The iris, as seen from the outside, determines our eye colour. A dark eye has many pigments, a light eye has fewer.
The anterior chamber of the eye is filled with a fluid, chamber water. Our cornea and the lens of our eye are nourished by the three cubic centimetres of this fluid produced daily. A sophisticated ventilation system ensures that fluid that is no longer needed drains smoothly.
The task of the iris is equivalent to the aperture of a camera. It dilates or shrinks the pupil. It can thus accurately regulate how many light rays reach the eye. It can expand or contract depending on the brightness. In extreme brightness, it can shrink down to 1.5 millimetres. On a dark night, it can open up to 8 millimetres.
The light rays then penetrate further, all the way to the lens of the eye. It has a diameter of about 9 millimetres and is 4 millimetres thick. It is enveloped by the so-called ciliary muscle. It is kind of like the zoom in a camera.
When you look at something in the distance, the muscle is relatively relaxed. But when you look at your watch, the muscle tightens. As a result, the lens curves and the refraction power increases.
This is measured in dioptres. Close objects can be reproduced very sharply by the curved lens. Experts call this process accommodation. The ability to accommodate is very well pronounced particularly in a person’s younger years. Twelve-year-olds with healthy eyes can see objects extremely sharply up to a distance of 8 centimetres. In one’s forties, this distance is 17 centimetres, and in one’s seventies, it’s only approx. one metre.
By the way: 95 percent of our visual acuity conglomerates in one tiny point in the retina. This mite with a diameter of only 2 millimetres is called the macula, or the yellow spot. In the fovea – experts call it the central macula - the number of cones responsible for colour vision is very high. The point of sharpest vision is thus located in our retina.
Also found in the retina are the rods that are responsible for vision at twilight and at night. The vitreous body fills in the rear part of the eye and consists of a gel-like substance. It protects and supports the retina and eye. It has an elastic structure and functions like a shock absorber if pressure is suddenly exerted on the eyeball.
The light finally strikes the retina. The most important components of the retina are the abovementioned rods and cones. They are photoreceptors, and everybody has just under 130 million of them.
The two have very different tasks: The over 120 million rods produce light/dark contrast and supply grey tones. The seven million cones, meanwhile, give us the gift of the beautiful colours of the world – but only when they receive ample light. All cats are still grey in the dark.
We humans and most primates have three different types of cones. This is why we can distinguish between red, green and blue. These are known as primary colours, and many other colours can be created from them. We humans perceive electro-magnetic radiation with wavelengths between 380 and 780 nanometres as light. For example: A wavelength between 650 and 700 nanometres triggers the signal "red."
The optic nerve starts behind the eye and extends to the vision centre in the brain. This is where sight information is relayed from the retina. The vision centre is located at the back of the head, where the sight information is processed. An upside-down image forms there, and the brain – or the cerebral cortex to be more precise- flips it. The actual sight process thus does not occur in the eye – it is our brain that provides the picture show.