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How the Eye Works
The Human Eye
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The eye is a complex and sensitive organ. Various eye diseases can limit you sight, are unpleasant or in some cases can permanently damage your eyesight. Regular visits to the optician are important, particularly in cases of complaints, so that treatment, when necessary, can be started early.
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Visual Perception
The eye belongs to the human sensory organs and is responsible for our visual perception. The most important components of the eye that enable us to see include the cornea, the iris, the lens, the retina, the macula and the optic nerve.
Bending the Light
The cornea is convex-arched and transparent. It protects the eye and at the same time is permeable to light. Its curvature ensures that the light is captured within the eye.The iris gives the eye its unmistakable colours and acts as an aperture. Depending on the light conditions it alters its opening (pupil) and in this way controls the amount of light passing through the pupil and into the eye.
The lens lies behind the iris and creates an image of what is seen on the retina. It is pliable and is given its shape by the ciliary muscle, by which it is surrounded. The curvature altered in this way sets the "focal length" of the lens, so that the eye can adapt to different sight distances.
Processing the Light
The retina lines the inside of the eye. It serves as a projection area. The image that is created on the retina is registered by nerve cells (cones and rods) and transformed to nerve impulses. The cones are responsible for seeing in colour, while the rods distinguish between light and dark.The macula, the so-called yellow spot, is located exactly on the rear side of the eye at the point furthest away from the lens. The macula is the point of sharpest sight; here there are only cones, and a great number of them.
The optic nerve conducts the signals that are gathered by the nerve cells in the retina, to the brain. And it is in the brain that the images we actually "see" are first created. Here the signals from the nerve cells are first interpreted, with the help of our memories and experiences.
The Function of the Healthy Eye
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How do we actually see?
Light falls onto the retina - via the cornea and the lens. In this process the focused light is projected onto a point on the retina. There are various ways in which the eye can alter the incidence of light within the eye.
The iris, for example, alters depending on the light conditions: in low light the pupil is widened, allowing more light to pass into the eye; in strong light the pupil contracts, so that less light passes into the eye.
The ciliary muscle, which surrounds the lens like a ring, shapes the lens to vary the curvature. In order to focus sharply on nearby objects, the lens is curved more strongly, which results in greater refraction of the light. On the other hand, if you want to focus sharply on distant objects the lens is more flattened, which means that the light is less strongly refracted.
Eye Injuries
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The eyes can be injured in many ways
Eye injuries can have a variety of causes. Typical examples include mechanical, optical, chemical, thermal or electrical injuries.
Mechanical Injuries
Mechanical injuries are caused by dust or solid bodies such as shavings and filings, splinters and grain that hit the eye. Dust irritates the eye and can cause inflammation. The main treatment in this case is the avoidance of any further irritation of the eyes, i.e. further exposure to dust. Solid bodies can penetrate the cornea and injure it. In this case medical treatment is required to remove the foreign body, to prevent possible inflammation and to avoid possible complications such as scarring or ulceration of the cornea.Optical Injuries
Optical injuries are caused by radiation. Exposure to intense or long-lasting radiation or differences in light can damage the eyes. Ultraviolet rays can cause ophthalmia. The complaints do not appear directly after exposure to the light, but several hours later. Inflammation of the cornea and conjunctivitis can follow. These can be painful, but they quickly subside.Light that is too bright (such as sunlight) can cause dazzling. This reduces one's vision for a shorter or longer period. This type of impaired vision is, however, normally only temporary.
Infrared radiation, during foundry work for example, can burn the retina. Anyone who is permanently exposed to infrared radiation is at risk of suffering a clouding of the lenses. Laser radiation can burn holes in the retina and thus cause lasting eye damage.
Chemical Damage
Damage can be caused by many types of chemicals, for example, alkaline or acid solutions. A drop of acid can lead to ulceration on the cornea and leave permanent scarring. Alkaline solutions are potentially more dangerous, a few drops can cloud the entire cornea permanently.Thermal Injuries
Thermal injuries can be caused by too much heat or cold. Heat gives rise to corneal irritation due to drying-out. Exposure to cold for a longer period triggers weepy eyes and can also lead to symptoms of frost injury.
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