Vision is one of the most valuable feelings of a person. And although the visual system is a relatively complex part of the brain, the process is due to a modest optical element: the eye. It forms images on the retina, where light is absorbed by photoreceptors. With their help, electrical signals are transmitted to the visual cortex for further processing.
The main elements of the optical system of the eye: cornea and lens. They perceive light and project it onto the retina. It is worth noting that the device of the eye is much simpler than that of cameras with multiple lenses created in its image. Despite the fact that only two elements play the role of lenses in the eye, this does not impair the perception of information.
Shine
The nature inherent in light also affects some characteristics of the optical system of the eye. For example, the retina is most sensitive in the central part to the perception of the visible spectrum corresponding to the radiation spectrum of the Sun. Light can be considered as a transverse electromagnetic wave. Visible wavelengths from approximately blue (400 nm) to red (700 nm) make up only a small fraction of the electromagnetic spectrum.
It is interesting to note that the nature of a light particle (photon) can also affect vision under certain conditions. Photon absorption occurs in photoreceptors according to the rules of a random process. In particular, the light intensity reaching each photoreceptor determines only the probability of photon absorption. This limits the ability of vision at low brightness and the adaptation of the eye to darkness.
Transparency
Artificial optical systems use transparent materials: glass or plastics with a refractive lock. Similarly, the human eye should form large-scale images with high resolution using living tissues. If the picture is projected onto the retina too blurry, fuzzy, the visual system will not work properly. The reason for this may be ocular and neural diseases.
Anatomy of the eye
The human eye can be described as a fluid filled quasispherical structure. The optical system of the eye consists of three layers of tissue:
- external (sclera, cornea);
- internal (retina, ciliary body, iris);
- intermediate (choroid).
In adults, the eye is represented by an approximate sphere with a diameter of 24 mm and consists of many cellular and non-cellular components obtained from ectoderm and mesoderm germinal sources.
Externally, the eye is covered with a stable and flexible tissue called the sclera, with the exception of the front, where a transparent cornea allows light to penetrate the pupil. The other two layers under the sclera are the choroid to provide nutrients and the retina, where light is absorbed by photoreceptors after imaging.
The eye is dynamic due to the action of six external muscles, allowing you to fix and scan the visual environment. The light entering the eye is refracted by the cornea: a thin transparent layer, free from blood vessels with a diameter of about 12 mm and a thickness of about 0.55 mm in the central part. An aqueous rupture film on the cornea guarantees the best image quality.
The anterior chamber of the eye is filled with liquid substance. The iris, two sets of muscles with a central opening, the size of which depends on compression, acts as a diaphragm with a characteristic color depending on the number and distribution of pigments.
The pupil is a hole in the center of the iris that controls the amount of light passing into the eye. Its size ranges from less than 2 mm in bright light to more than 8 mm in the dark. After perception of light by the pupil, the crystalline lens in combination with the cornea form images on the retina. A crystalline lens can change its shape. It is surrounded by an elastic capsule and attached by zonals to the ciliary body. The action of muscles in the ciliary body allows the lens to increase or decrease its power.
Retina and cornea
There is a central depression in the retina where the largest number of receptors are located. Its peripheral parts give lower resolution, but specialize in eye movement and object detection. The natural field of view is quite large compared to the artificial one and is 160 × 130 °. A macula is located nearby and functions as a light filter, presumably protecting the retina from degenerative diseases by screening out blue rays.
The cornea is a spherical section with a radius of anterior curvature of 7.8 mm, a posterior of 6.5 mm and a non-uniform refractive index of 1.37 due to the layered structure.
Eye Size and Focus
The srednestatic eye has a total axial length of 24.2 mm, and distant objects are focused exactly in the center of the retina. But the deviations in the size of the eye can change the situation:
- myopia, when images are focused in front of the retina,
- farsightedness when this happens behind her.
The functions of the optical system of the eye are also violated with astigmatism - improper curvature of the lens.
Retinal Image Quality
Even when the optical system of the eye is perfectly focused, it does not reproduce the perfect image. This is influenced by several factors:
- diffraction of light in the pupil (blur);
- optical aberrations (the larger the pupil, the worse the visibility);
- dispersion inside the eye.
The specific forms of the lenses of the eye, variations in the refractive index, and geometry are disadvantages of the optical system of the eye compared to artificial counterparts. The normal eye has at least six times lower quality, and each creates an original point image depending on the aberrations present. So, for example, the perceived shape of the stars will be individual for everyone.
Peripheral vision
The central field of the retina gives the greatest spatial resolution, but the less sharp peripheral part is also important. Thanks to lateral vision, a person can navigate in the dark, distinguish between the factor of movement, and not the moving object itself and its shape, navigate in space. Peripheral vision prevails in animals and birds. Moreover, for some of them, the viewing angle is all 360 ° for higher chances of survival. Visual illusions are calculated on the features of peripheral vision.
Total
The optical system of the human eye is simple and reliable and perfectly adapted to the perception of the world. Although the quality of the visible is lower than in advanced technical systems, but it meets the requirements of the body. There are a number of compensation mechanisms in the eyes that leave some of the potential optical limitations negligible. For example, the large negative effect of chromatic defocusing is eliminated by appropriate color filters and band-pass spectral sensitivity.
In the last decade, the possibility of correcting eye aberrations using adaptive optics has been actively discussed. Currently, it is technically possible in the laboratory using corrective devices, for example, intraocular lenses. Correction can restore the ability to vision, but there is a nuance - the selectivity of photoreceptors. Even if sharp images are projected onto the retina, the smallest letter that needs to be perceived will require several photoreceptors for proper interpretation. Images of letters smaller than the corresponding visual acuity will not be different.
However, the main visual disturbances are mild aberrations: defocusing and astigmatism. These cases are easily remedied by various technological developments since the thirteenth century, when cylindrical lenses were invented. Modern methods involve the use of contact and intraocular lenses or laser refractive surgery procedures to edit the structure of the patient's optical system.
The future of ophthalmology seems promising. Photonics and lighting technology will play a key role in it. The use of advanced optoelectronics would allow new prostheses to restore farsighted eyes without removing living tissue, as is happening now. New optical coherence tomography could provide full-scale three-dimensional visualization of the eye in real time. Science does not stand still, so that the optical system of the eyes allows each of us to see the world in all its glory.