The eye has the role of providing information – in the form of colorful images – about the depth, distance and movement of objects. Moving it up, down and sideways, we see most of the environment around us.
Through the eyes we receive the most information about the outside world. According to the calculations of a researcher, 80% of the memories we keep are recorded by sight.
If we look at a camera, we can better understand how our eye works. The anterior portion of the eye functions as an optical lens, as does the glass lens of the camera. The optical lens is a body with one or two curved surfaces made of a transparent material. The light penetrated by such a body is refracted.
The dark portion of the center of the eye, the pupil, adjusts the amount of light received. When the light is dim, the pupil will be larger, if it shrinks, it will leave a reduced amount of light, as in the case of the aperture behind the lens of the camera. The layer of the depth of the eyeball, the retina, corresponds to the photographic film.
Our eye is much more complex than the camera. With the help of cameras we can only fix the images from the outside world on a photographic film, while animals and humans can interpret the information on the retina and act according to the information received.
This is possible due to the fact that through the optic nerve the eye has a connection with the brain. The optic nerve attaches to the eyeball at its posterior portion through a small pedicle. Optical information intercepted by the retina is transmitted through the optic nerve to the brain. The information is transmitted in the form of electrical impulses in the brain, which decode them.
The two eyes look at slightly different objects from the outside world, so the information sent to the brain is somewhat different. But our brain “learns” from the first days to assemble the two images, so we do not see objects in duplicate. By brainstorming the two images, the brain deduces the location of objects in space and the distance at which they are – this makes it possible to see three-dimensional.
The brain transforms the seen image from the turned position into the right position. The light is refracted in the lens and will project on the retina an inverted image. Because we cannot look at the world all our lives, the brain “reads” the image and immediately returns it to the right position. To learn this is a long time coming, which is why babies see the world turned upside down at first.
The human eye is like a ball. In front in the middle there is a transparent layer, slightly prominent, the cornea. This is related to the layer that forms the white of the eye and covers around the eyeball – sclera. The margins of the sclera are provided with a rich network of blood vessels.
The cornea is the first medium of light refraction – the optical lens – through which light passes. Its position and shape cannot be changed, and as a result, neither focal length.
Under the cornea is the iris. It gives the color of the eye – most often goats, blue or green. The iris is actually a muscular disc, with a hole in the center: the pupil. The light penetrates inside the eye through the pupil.
The aqueous humor between the cornea and the iris helps maintain corneal cleanliness and remove germs.
Immediately after the iris comes the crystalline, the second refractive medium, which is however mobile and elastic. It is fixed by the choroid processes. The shape of the lens can be changed with the help of the muscles in the choroid bodies. When we look at a distant object, these muscles relax, the lens widens and flattens. If we look at a nearby object, the lens becomes convex.
The space behind the lens, the posterior chamber, is filled with a gelatinous substance – the vitreous humor. The light that is refracted by the cornea and crystalline must pass through the vitreous body, after which it touches the retina, which covers the bottom of the eye.
The retina contains approximately 130 million photosensitive cells – cones and sticks. The sticks are very light sensitive, but except for the blue and green colors, they cannot differentiate the colors. Cones can distinguish colors and increase the clarity of the image, but they are not functional in low light conditions. This is the explanation of the fact that under the twilight light we do not see clearly and the colors “disappear”, everything appears in shades of blue or green. In such situations, only the sticks work. The French call that time of day “l’heure bleu”, which is the blue hour.
In very strong light only the cones work. When the light decreases in intensity, the sticks reactivate, but the process takes some time. When you enter the street in a dimly lit room, your eye needs to adjust to the dim light, and when you go out again in the sun, you are “blind” for a few seconds.
The blindness caused by certain diseases of the retina comes from the deterioration of the sticks and cones, which give up after a certain time. The researchers are trying to stimulate and reactivate the affected cones and sticks with the help of electrodes. Another possibility is the implantation of cones and sticks taken from embryonic tissues, thus restoring the retinal function.
The cones are crowded into the posterior portion of the retina, in the place called the yellow spot. Most sticks are located outside the yellow spot, along with a few cones, less numerous.
Near the yellow spot, also on the retina, is the site of insect optic nerve. In this place there are no photosensitive cells, the light beams that arrive here are not intercepted. That point is called the blind spot, and it exists in both eyes.
The image that is projected in the central portion of the retina appears the clearest, which is why it is important for the eyeballs to be mobile, being able to orient the gaze towards the object pursued. Due to the six muscles that attach to the sclera, the eyeball has very high mobility.
The eye is protected from all sides. It is housed in the orbit of bones, lined with adipose tissue. During hits, different accidents, the orbit is more frequently affected than the eye itself. The anterior face of the eye and the inner portion of the eyelids is covered by a transparent layer – conjunctiva – it protects and cleans, practically “warms up in tears” the entire anterior surface of the eye. The tear is produced by the tear glands (Harder) located in the outer corner of the eye orbit, and is driven through the tear duct in the inner corner of the eye, into the nasal cavity. If there is dust, or dirt in the eyes, the tear glands begin to produce more tears.
The eyelid conjunctivitis (the one that lines the inner face of the eyelids), cleanses the eye at every blink. The eyelids protect the eyes from light too strong, or from boiling particles carried by the wind, which could enter the eye. The genes also have a protective role against foreign particles. Not even the eyebrows are just simple ornaments: they protect the eye from the breathable drops that flow from the forehead.
The most common vision defects are myopia, respectively hyperopia. Myopia cannot clearly see distant objects, while hypermetropia forms a blurry image of nearby objects.
These defects are almost without exception the consequence of changing the shape of the eyeball. For a perfect view, the eyeball must be spherical. The eyeball of the myopia is elongated horizontally, and the hyperopia is shortened. With glasses or contact lenses, both vision defects can be corrected. More recently, researchers are trying to correct myopia by flattening the cornea.
This intervention, called “radical keratotomy” consists of making incisions in the form of wheel weights. As soon as these incisions heal, the cornea will be flatter. The operation can also be performed with a laser. Myopia severity data is entered into a computer, which will calculate the amount of cornea that needs to be excised to remedy myopia.
Changing the shape of the eyeball can also cause another defect of vision: astigmatism. This condition usually accompanies myopia or hyperopia. The curvature of the healthy cornea is uniform, just like a soccer ball. In some, however, the curvature of the cornea is more like a rugby ball and has as a result a difficulty in focusing the objects.
The crossed eyes do not look in the same direction: often both eyes look inwards or outwards, but there are cases when they are oriented up or down (convergent, divergent, respectively height squint). Most often the cause is that one of the oculomotor muscles is weak.
In childhood, the most common vision disorders are caused by squinting or different accidents. At an older age, other eye disorders, such as glaucoma or cataracts, may occur.
The cause of glaucoma is the increase in the aqueous amount that accumulates between the iris and the cornea, which exerts a high pressure and an accentuated pain. The vision can become blurred, the disease leading to blindness, if left untreated. In some cases, a small laser incision is made on the corneas to reduce the pressure in the anterior chamber of the eye.
Cataract is the opacification of the lens. The patient has the impression that he looks at the world through a glass, which freezes gradually. Cataracts develop over time and are not accompanied by pain. The operation consists of removing the opacified lens with the help of an ultrasound device (high frequency sounds, imperceptible to our ears), and its replacement with an artificial one, of plastic material.
The pupil is the opening in the center of the layer that gives the color of the eye: the iris. The amount of light entering the pupil is regulated by the iris. In strong light, the iris contracts. The pupil will shrink, leaving only a small amount of light on the surface of the retina. At twilight, the iris relaxes, the pupil dilates and allows more light to penetrate the eyes. The pupil can also dilate under the influence of strong emotions (love, fear).
If you are short-sighted, do not see the distant images clearly, because the light beams meet before the plane of the retina. This can happen if the eyeball is elongated horizontally, or if the lens refracts too much light. Myopia can be corrected with the concave lens, so the light beams will meet further, at the level of the retina.
If you are hypermetropic, the objects you see close up are unclear. If the eyeball is too short on the horizontal plane, or the lens does not refract light enough, then the light beams from nearby objects do not meet up to the level of the retina. The problem can be corrected with the convex lens, because it approximates the light beams, which will meet at the level of the retina.
Particularly three-dimensional effects can be obtained in the films, if the images are made in two, slightly different versions – one in green, the other in red – after which the two images overlap. Viewers can watch the movie with special glasses: one of their “lenses” is red, and the other is green, so each eye will have only part of its intended image.