Light and dark

The eye constantly adapts as there is more or less light. That happens through more mechanism, like the regulation of the size of the pupil. At glaring light the pupil will be small, so that less light can enter the retina. In the darkness the pupil widens and can reach a maximum amount of light on the retina.

As the light reduces, especially the rods become more active. They provide us to see in the twilight as well. With some people this mechanism doesn't work well: they are night-blind.

The way of the light

The light hurls into our eye with a dizzy speed of almost 300.000 kilometres per second. The retina is continuously bombed by slanting rays of light. To protect them from damage, many defensive mechanisms are built-in. They will provide that the right amount of light enters the retina.

The light passes the cornea and the liquid of the foremost chamber of the eye. Then it arrives at the opening in the iris: the pupil. The muscles react immediately: when there is much light, the pupil will become smaller, when there is less light, the pupil will widen. That whole process can last a few seconds. You are well aware of it when you get out of a dark room into the glaring sunlight, or when you suddenly come out a of the dark into a place where there's much light: at a moment you will be blinded by the light.

The lens

The light which enters the pupil falls on the lens. The lens gets more spherical and flatter; therefore the image enters the focus as clearly as possible. The focus is situated at the back of the eyeball, where the rods and cones are. The light will be converted in electrical streams, which are being sent via nerve cells and the optic nerves to the brains.

The creation of the picture

The real image is formed, as told before, in the brains. This too is a complicated process, at which the information of the two eyes will be combined and joined together with fragments of the memory. The brains decide eventually what you see.

Short-sighted and far-sighted

Who is myopic, can see very clearly closeby but has a difficulty to distinguish something further away. That's because the cornea and the lens are too spherically compared in relation of the length of the eye. The rays of light are bent too strongly and the clear image enters in front of the retina.

When you are far-sighted, you experience the reverse. What's far away will be seen clearly, closeby will be blurred. The lens is too flat in relation to the length of the eye. The rays of light are being bent less, so that the clear image enters behind the retina.

To correct these two defects, the bending of the system of the lenses can be changed. Glasses or contact-lenses will take care that the light will be bent in the right direction. A spherical lens corrects short-shightness, a concave lens far-sightness.

Short-sightness and far-sightness are hereditary. Children with parents, who have these problems, have the chance to get it themselves as well.

At a certain age the sight of many people will be limited. Mostly at the age of forty. An older lens is less flexible and bends less easy. The sight further away will be fine, but closeby will become harder.

Applet