The optical system of the eye

The human eye is a complex optical system consisting of the cornea, the anterior chamber fluid, the lens, and the vitreous body. The refractive power of the eye depends on the magnitude of the radii of curvature of the anterior surface of the cornea, the anterior and posterior surfaces of the lens, the distances between them, and the refractive indices of the cornea, lens, aqueous humor, and vitreous body. The optical power of the posterior surface of the cornea is not taken into account, since the refractive indices of the corneal tissue and the anterior chamber fluid are the same (as is known, refraction of rays is possible only at the boundary of media with different refractive indices).

Conventionally, it can be considered that the refractive surfaces of the eye are spherical and their optical axes coincide, i.e. the eye is a centered system. In reality, the optical system of the eye has many errors. Thus, the cornea is spherical only in the central zone, the refractive index of the outer layers of the lens is less than the inner ones, the degree of refraction of rays in two mutually perpendicular planes is not the same. In addition, the optical characteristics in different eyes differ significantly, and it is not easy to determine them accurately. All this complicates the calculation of the optical constants of the eye.

To evaluate the refractive power of any optical system, a conventional unit is used - diopter (abbreviated - dptr). For 1 dptr, the power of a lens with a main focal length of 1 m is taken. Diopter (D) is the reciprocal value of the focal length (F):

D=1/F

Therefore, a lens with a focal length of 0.5 m has a refractive power of 2.0 dptrs, 2 m - 0.5 dptrs, etc. The refractive power of convex (converging) lenses is indicated by the plus sign, concave (diverging) lenses - by the minus sign, and the lenses themselves are called positive and negative, respectively.

There is a simple method by which you can distinguish a positive lens from a negative one. To do this, you need to place the lens at a distance of several centimeters from the eye and move it, for example, in the horizontal direction. When looking at an object through a positive lens, its image will move in the direction opposite to the lens movement, and through a negative lens, on the contrary, in the same direction.

To carry out calculations related to the optical system of the eye, simplified schemes of this system are proposed, based on the average values of optical constants obtained by measuring a large number of eyes.

The most successful is the schematic reduced eye proposed by V. K. Verbitsky in 1928. Its main characteristics are: the main plane touches the apex of the cornea; the radius of curvature of the latter is 6.82 mm; the length of the anterior-posterior axis is 23.4 mm; the radius of curvature of the retina is 10.2 mm; the refractive index of the intraocular medium is 1.4; the total refractive power is 58.82 diopters.

Like other optical systems, the eye is subject to various aberrations (from the Latin aberratio - deviation) - defects of the optical system of the eye, leading to a decrease in the quality of the image of an object on the retina. Due to spherical aberration, rays emanating from a point source of light are collected not at a point, but in a certain zone on the optical axis of the eye. As a result, a circle of light scattering is formed on the retina. The depth of this zone for a "normal" human eye ranges from 0.5 to 1.0 diopters.

As a result of chromatic aberration, the rays of the short-wave part of the spectrum (blue-green) intersect in the eye at a shorter distance from the cornea than the rays of the long-wave part of the spectrum (red). The interval between the foci of these rays in the eye can reach 1.0 Dptr.

Almost all eyes have another aberration caused by the lack of ideal sphericity of the refractive surfaces of the cornea and lens. Asphericity of the cornea, for example, can be eliminated with the help of a hypothetical plate, which, when placed on the cornea, turns the eye into an ideal spherical system. The absence of sphericity leads to uneven distribution of light on the retina: a luminous point forms a complex image on the retina, on which areas of maximum illumination can be distinguished. In recent years, the influence of this aberration on maximum visual acuity has been actively studied even in "normal" eyes with the aim of correcting it and achieving so-called supervision (for example, with the help of a laser).

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Formation of the optical system of the eye

An examination of the visual organ of various animals in the ecological aspect testifies to the adaptive nature of refraction, i.e. to such a formation of the eye as an optical system that provides the given animal species with optimal visual orientation in accordance with the characteristics of its life activity and habitat. Apparently, it is not accidental, but historically and ecologically conditioned that humans have predominantly a refraction close to emmetropia, which best ensures clear vision of both distant and close objects in accordance with the diversity of their activities.

The regular approach of refraction to emmetropia observed in most adults is expressed in a high inverse correlation between the anatomical and optical components of the eye: in the process of its growth, a tendency to combine a greater refractive power of the optical apparatus with a shorter anterior-posterior axis and, conversely, a lower refractive power with a longer axis is manifested. Consequently, eye growth is a regulated process. Eye growth should be understood not as a simple increase in its size, but as a directed formation of the eyeball as a complex optical system under the influence of environmental conditions and the hereditary factor with its species and individual characteristics.

Of the two components - anatomical and optical, the combination of which determines the refraction of the eye, the anatomical is significantly more "mobile" (in particular, the size of the anterior-posterior axis). It is mainly through it that the body's regulatory influences on the formation of the refraction of the eye are realized.

It has been established that newborns' eyes, as a rule, have weak refraction. As children develop, refraction increases: the degree of hypermetropia decreases, weak hypermetropia turns into emmetropia and even myopia, emmetropic eyes in some cases become nearsighted.

During the first 3 years of a child's life, there is intensive growth of the eye, as well as an increase in the refraction of the cornea and the length of the anteroposterior axis, which by the age of 5-7 years reaches 22 mm, i.e., is approximately 95% of the size of an adult eye. The growth of the eyeball continues until 14-15 years. By this age, the length of the eye axis approaches 23 mm, and the refractive power of the cornea - 43.0 diopters.

As the eye grows, the variability of its clinical refraction decreases: it slowly increases, i.e. shifts towards emmetropia.

In the first years of a child's life, the predominant type of refraction is hyperopia. As age increases, the prevalence of hyperopia decreases, while emmetropic refraction and myopia increase. The frequency of myopia increases especially noticeably, starting from 11-14 years, reaching approximately 30% at the age of 19-25. The share of hyperopia and emmetropia at this age is approximately 30 and 40%, respectively.

Although the quantitative indicators of the prevalence of individual types of eye refraction in children, given by different authors, vary significantly, the above-mentioned general pattern of change in eye refraction with increasing age remains.

At present, attempts are being made to establish average age norms of eye refraction in children and to use this indicator to solve practical problems. However, as the analysis of statistical data shows, the differences in the magnitude of refraction in children of the same age are so significant that such norms can only be conditional.

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