What is the shape of my eye

eye

How does the eye work?

The function of the eye consists in the optical perception of our environment. This “seeing” is a complex process: the eye must first convert incident light into nerve stimuli, which are then passed on to the brain. However, the human eye only perceives electromagnetic rays with a wavelength of 400 to 750 nanometers as "light". Other wavelengths are invisible to our eyes.

Considered in detail, the process "seeing" two functional units involved: the optical (dioptric) apparatus and the receptor surface of the retina. In order to see optimally, the eye must be able to adapt to different lighting conditions (adaptation) and to switch between distance and near vision (Accommodation). You can read more about this in the following sections.

Functional unit optical apparatus

The optical device (also known as a dioptric device) ensures that the light rays falling into the eye are refracted and bundled and hit the retina. Its components include:

  • Cornea
  • Eye lens
  • Vitreous
  • Aqueous humor

The cornea has the greatest refractive power of the eye (+43 diopters). The other structures (lens, vitreous humor, aqueous humor) are less able to break the light rays. In summary, this results in a total refractive power of normally 58.8 diopters (applies to the eye at rest and focused on distance vision).

Functional unit retina

The light rays bundled by the optical apparatus hit the receptor surface of the retina and create a scaled-down and upside-down image of the object being viewed. Suppositories and rods - into electrical impulses, which are then passed on from the optic nerve to the cerebral cortex. This is where the perceived image is created.

adaptation

The eye must look at each other during the process of seeing different light intensities to adjust. This so-called light-dark adaptation takes place via various mechanisms, including above all:

  • Change in pupil size
  • Alternation between rod and cone vision
  • Change in rhodopsin concentration

Change in pupil size

The iris of the eye changes the pupil width in adaptation to the light intensity:

When stronger, brighter light hits the eyeball, the pupil narrows so that less light falls on the delicate retina. Too much light would be blinding. In contrast, when the light intensity is low, the pupil expands so that more light hits the retina.

A camera works in a similar way: The diaphragm here corresponds to the iris, the aperture to the pupil.

Alternation between rod and cone vision

The retina can adapt to different light conditions by switching between rod and cone vision:

In the Twilight and darkness switches the retina to seeing with the rods. This is because these are much more sensitive to light than the cones. However, you cannot see any colors in the dark because the rods are unable to do so. In addition, you cannot see clearly at night. At the point of sharpest vision in the retina - the fovea centralis - there are no rods, but only all around in the rest of the retina.

At the bright day on the other hand, the retina switches to cone vision. The cones are responsible for color perception - that's why you can see colors during the day. In addition, sharp vision is then also possible because the cones are particularly close at the point of sharpest vision (pit of vision), while they become rarer towards the edge of the retina.

Change in rhodopsin concentration

Rhodopsin (visual purple) is a Pigment in the rodswhich is made up of two chemical components: opsin and 11-cis-retinal. With the help of rhodopsin, the human eye can distinguish between light and dark. It does this by converting light stimuli into electrical signals - a process that Light transduction (photo transduction) is called. It works like this:

When a light stimulus (photon) hits the rhodopsin, its component 11-cis-retinal is converted into all-trans-retinal. As a result, rhodopsin is converted into metarhodopsin II in several steps. This sets a signal cascade in motion, at the end of which an electrical impulse is created. This is transmitted to the optic nerve by certain nerve cells in the retina (bipolar cell, ganglion cell), which are connected to the rods.

After exposure - i.e. in twilight and darkness - the rhodopsin regenerates so that it is available again in larger quantities. This increases the sensitivity to light again (dark adaptation).

The degradation of rhodopsin (when exposed to light) takes place quickly, its regeneration (in the dark) much more slowly. Therefore, changing from light to dark takes much longer than changing from dark to light. It can take up to 45 minutes for the eye to "get used" to the darkness.

Accommodation

The term accommodation generally stands for the functional adaptation of an organ to a specific task. In connection with the eye, accommodation refers to the adaptation of the refractive power of the eye lens to objects at different distances.

The lens of the eye is suspended in the eyeball on the radiation body (ciliary body), which contains the ciliary muscle. From this, fibers pull into the lens of the eye, the so-called zonular fibers. If the tension of the ciliary muscle changes, this also changes the tension of the zonular fibers and subsequently the shape and thus the refractive power of the eye lens:

Long-distance accommodation

When the ciliary muscle is relaxed, the zonular fibers are taut. Then the eye lens is drawn flat at the front (the back remains unchanged). The refractive power of the lens is then low: light rays falling into the eye are refracted and united on the retina in such a way that we distant objects sharp see.

The farthest point that can still be seen clearly becomes Far point called. In the case of people with normal vision, it is infinite.

Remote adjustment of the eye also means that the pupil dilates and the eyes diverge.

Near accommodation

When the ciliary muscle contracts, the zonular fibers relax. Due to its inherent elasticity, the lens then changes to its rest position, in which it is more curved. Your refractive power is then higher. Thus, light rays incident on the eye are refracted more strongly. Appear in the episode close objects sharp.

As Near point is the term used to describe the shortest distance at which something can still be seen clearly. In normally sighted young adults, it is about ten centimeters in front of the eyes.

With closer focus, the pupil also narrows, which improves the depth of field, and both eyes converge.

Accommodation resting point

In the resting state, if there is no accommodation stimulus at all (e.g. in absolute darkness), the ciliary muscle is in an intermediate position. As a result, the eye is focused at a distance of about one meter.

Accommodation width

The range of accommodation is the area in which the eye can change its refractive power when switching between distance and near vision. A young person's range of accommodation is around 14 dioptres: their eyes can see objects at a distance of between seven centimeters and “infinitely” sharp, whereby the ophthalmologist understands “infinite” as a distance of at least five meters.

From the age of 40 to 45, the ability to accommodate - i.e. the ability of the eye lens to change its shape and thus its refractive power - steadily decreases. The reason: the rigid core of the lens becomes larger with age, while the deformable lens cortex becomes less and less. Finally, as people get older, the range of accommodation can drop to around one diopter.

So naturally, as people get older, they become increasingly farsighted. These age-related, inevitable farsightedness is called presbyopia).