Our Eyes - Our Windows to the World

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 color of the bedspread, the green leaves on the tree outside the window. Vision supplies us with 80% of our impressions of our surroundings.

Our eyes function like a camera in which numerous parts work together perfectly.

The average human eye has a diameter of about an inch and weighs just about 7.5 grams. It is composed of 6 grams of water and 1.5 grams of cell tissue --low material costs 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.

How the optics of the eyes work

Regardless of whether you're looking at the latest fashion magazine or watching a football game on television, you see because light rays penetrate your 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 iris, as seen from the outside, determines our eye color. A dark eye has many pigments, a light eye has fewer.

The anterior chamber of the eye is filled with a fluid called "chamber water." Our cornea and the lens of our eye are nourished by the three cubic centimeters (about 1/10 of an ounce) 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 millimeters. On a dark night, it can open up to 8 millimeters.

How does vision actually work?

The light rays then penetrate further, all the way to the lens of the eye. It has a diameter of about 9 millimeters and is 4 millimeters thick. It is enveloped by the so-called ciliary muscle, which functions 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.

Refraction power is measured in diopters. Close objects can be reproduced very sharply by the curved lens. Experts call this process accommodation . The ability to accommodate is very strong in a person's younger years. Twelve-year-olds with healthy eyes can see objects extremely sharply up to a distance of 3 inches . In one's forties , this distance increases to about 7 inches , and in one's seventies, it's up to three feet.

95 percent of our visual acuity is generated on one tiny point in the retina. This area, with a diameter of only 2 millimeters, is called the macula, or the yellow spot. In the fovea - experts call it the central macula - the number of cones responsible for color vision is very high. The point of sharpest vision is thus located in the 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.

And what happens in the retina?

The light finally strikes the retina. The most important components of the retina are the 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 colors 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 colors, and many other colors can be created from them. We humans perceive electro-magnetic radiation with wavelengths between 380 and 780 nanometers as light. For example, a wavelength between 650 and 700 nanometers triggers the signal "red."

The optic nerve starts behind the eye and extends to the vision center in the brain. This is where sight information is relayed from the retina. The vision center 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.


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