Vision is an extremely complex functional mechanism that people take for granted. It begins with the passage of a stream of light from objects through our eyes, with the conversion of the energy of light quanta into a nerve impulse carrying information to the brain. Our eyes are organs like the heart, kidneys, and skin (our largest body part). The ocular anatomy is incredibly sophisticated because each eye consists of more than two million components. The eye is one of the most important sense organs, and like any other organ, it is very complex. The human eye can receive and process millions of pieces of data per second.
From a physics course, one may recall lenses – devices made of transparent material with a refractive surface that, depending on their shape, can collect or scatter light falling on them. It is to lenses that we owe the fact that there are cameras, video cameras, telescopes, binoculars, and, of course, contact lenses and glasses that people wear. The human eye is the same lens, more precisely, a complex optical system consisting of several biological lenses: cornea, anterior chamber of the eye, iris, pupil, lens, vitreous, and retina.
The perception of objects is part of a complex process: before seeing anything, a sequence of separate steps is performed in the eye and brain. The retina genicular-striate pathway begins in the eye and passes into the brain. Simply put, vision works as follows: light from the environment penetrates the human eye and collects on the cornea, which leads to the initial visual perception. Then each eye directs the resulting image to the brain through the optic nerve, which leads to the image occurrence. Light is the necessary basis for everything a human sees because, in total darkness, they are practically blind.
Thus, to see any object, at least a little light must fall on it. Then the visual apparatus processes the light, and if people look at a dog, the light reflected from it enters their eyes while first, the rays of light pass through the conjunctiva and cornea; then, they penetrate the anterior chamber and the pupil. After that, the beams enter the eye lens, where they are stored and transmitted to the photosensitive retina. Visual information is collected and sorted here: the rods allow one to distinguish dark from light, and cones are responsible for clarity and perception of color. This information is transmitted to the optic nerve, through which it enters the brain, where it is again analyzed, defined, and combined to create the picture that humans see.
Observing objects in good light (photopic or day vision) is a task performed by cones (sensitive cells responsible for color vision). The pupil also takes part in daytime eyesight: the higher the brightness, the smaller the pupil. It adjusts to different intensities of light and controls the amount of light penetrating the eye – it is called adaptation. Normally, ophthalmologists recommend wearing sunglasses and darkened lenses to protect one’s eyes from sunlight.
Moreover, healthy eyes can automatically switch between near and far vision and see objects in both cases. The dynamic ability to observe items at different distances is called accommodation which is based on the elasticity of the eye’s lens. In the absence of violations, it can change its shape and, therefore, adapt in such a way as to see near and far objects. In a normal state, the eye’s lens is flat and long, which allows one to see distant objects. However, when staring closely at the object, the lens becomes convex: it turns to the close range and enables one to get a clear view of near objects. Accommodation is normally used when the images of objects projected onto the fovea seem fuzzy.
There is also a night vision termed scotopic vision; healthy eyes take about 25 minutes to get used to the darkness. The less light there is the more active the rods (the sensitive cells of the eye responsible for distinguishing light and darkness) work. To let in the maximum amount of light, the pupils dilate. Healthy eyes do not experience problems when changing the illumination. Hereditary disorders, specific drugs, and a lack of vitamin A can lead to poor vision in the dark or twilight. This problem often occurs with people who wear glasses because, in poor light, the pupils should expand more. As a consequence, there is a limitation of spatial vision, as well as eye strain due to reflected light and low contrast.
Consequently, the mechanism of forming an image of reality in our brain is not only optics and chemical reactions occurring on the retina. The most important role in creating this picture is played by our brain and not only by the visual cortex. This part makes the figures voluminous, separates them from the background, and paints them in the right colors, but also the rest of the departments are responsible for vital functions.