The refractive system of the eye

Refractive Eye Apparatus
The refractive system of the eyeball (bulbus oculi) consists of four basic parts (cornea, lens, aqueous humor, vitreous), which give it an optical power of approximately +60D, i.e. the eye behaves like clutch. A properly functioning (emmetropic) eye concentrates the incident rays exactly on the surface of the retina, while the properties of the eye apparatus result in the image being inverted, reduced and real. The position of the eyes ensures that, with flawless functioning of the refracting apparatus, when looking directly, the rays spread to the so-called yellow spot (macula lutea) on the retina, where the highest concentration of light-sensitive cells (cones) is located - i.e. to the place of the highest discernment. Even in this location, however, we are not able to see an object at a smaller angular distance than 3.10-4 rad – corresponding to one minute of arc.

Cornea
The almost spherically convex cornea (cornea) represents the ventral surface part of the eyeball passing into the sclera. In an adult, its width reaches 0.8 mm, the refractive index is 1.37. Since the cornea 'communicates with the environment'' (air) with a refractive index of 1, the refraction of light at the transition between the air and the cornea is most noticeable. The cornea alone makes up roughly 70% of the entire optical power of the eye - i.e. +42 to +43 D.

Lens
The up to 4 mm thick lens (lens crystallina) of biconvex shape is made of a heterogeneous clear mass with an average refractive index of 1.42. The optical power of the lens, which is located between the cornea and the vitreous, is not static (+16 to +20 D), on the contrary, its value is adjusted by means of the corpus cilliare, on which the lens is fixed. By loosening the ciliary body, the lens will shrink and therefore also increase its optical power (i.e. when focusing at close range). The opposite phenomenon (tension of the lens) provides better vision for up to infinitely distant objects (optical power decreases).

Vitreous
Although the clear, jelly-like vitreous (corpus vitreum) fills more than two-thirds of the volume of the eyeball, it does not significantly contribute to the overall optical power. It creates an environment for the propagation and impact of already refracted rays on the retina. It also provides eye protection and maintains intraocular pressure.

Chamber water
The thick intraocular (chamber) fluid, found in the anterior and posterior chamber of the eye, forms part of the refractive apparatus of the eye, but it almost does not affect the optical power of the eye. It maintains the necessary intraocular pressure and nourishes the eye.



Accommodation of the eye
In order for the observed objects to be displayed correctly on the retina - i.e. for sharp vision - it is necessary that the optical power of the eye changes with the different distance of the observed object. This ability is provided by the lens, which is stretched and loosened thanks to its suspension on the ciliary body. However, the function of the ciliary body and the lens also weakens with wear and tear and the age of the organism, and thus the so-called accommodating range of the eye is reduced. The accommodative range of the eye refers to the optical range between the near and far point.

Near point
The near point (punctum proximum) can be characterized as the closest point at just such a distance at which the observed object can be seen sharply at the highest accommodation - i.e. at the highest contraction of the elastic lens and maximum release of the ciliary apparatus. The ability to focus at close range therefore largely depends on the elasticity of the lens, which decreases with age. In a child, the distance of the near point is around 10 cm, in an adult up to around 25 cm. This distance is called the conventional visual distance (with an accommodative range of 4 D). In old age, the near point moves further away, causing difficulties with close focusing - the so-called "presbyopia". It can also happen that the ability to accommodate the eye completely disappears, and therefore the eye does not change its optical power - it reaches a zero range of accommodation.

Far Point
The far point (punctum remotum) is located at the greatest distance at which we can see the observed object sharply (without accommodation). At this point, the lens is maximally stretched (it has low optical power). In a healthy eye, the distant point is found at a distance of 5 m (referred to as infinity).

Defects of the eye
In many cases, sharp vision is disturbed by non-physiological phenomena - eye defects that are more or less easily corrected. Defects of the refractive apparatus of the eye include nearsightedness (myopia), farsightedness (hypermetropia), farsightedness (presbyopia) and astigmatism. Eye defects are thoroughly discussed in the article Refractive defects.