Eye (biophysics)

The eye is the sensory organ of sight, it consists of a photosensitive layer containing light-sensitive cells and an optical system that directs the rays so that they fall on the retina. The eye is approximately spherical in shape with a radius of 12 mm.

The refractive system of the eye
For more information go to: Refractive system of the eye.

Light passes through the following parts of the eye before it hits the photoreceptors:
 * 1) Cornea - refractive index 1.377;
 * 2) anterior chamber of the eye – filled with aqueous humor with a refractive index of 1.336;
 * 3) iris, or an opening in the iris called the pupil ;
 * 4) posterior chamber of the eye – between the posterior surface of the iris and the ciliary apparatus, n = 1,336;
 * 5) lens – n = 1.42, which can change the curvature and thus the optical power through the ciliary apparatus;
 * 6) vitreous ;
 * 7) retina- a person has a so-called inverse type of retina, i.e. the photoreceptors are only on the side away from the lens.

The optical power of a healthy eye as an optical system is infinite for an object, it increases during accommodation.

Photosensitive cells
For more information go to: Photosensitive cells and their functions.

Photosensitive cells of the retina are cells that create nerve stimulation based on the absorption of a photon arriving at the retina. These cells are of two types: rods and cones.

Suppositories are sensitive to light of different colors, i.e. different wavelengths, different intensities and different color saturations. They are the first neurons of the retina. They ensure photopic vision, are responsible for visual acuity. They are found most abundantly in the central fovea ( fovea centralis ), which is a small pit in the macula. Towards the periphery of the retina, their density gradually decreases. In total, we find 6 million cones on the retina. We distinguish 3 types of cones, which can only be distinguished by the pigment in the cytoplasm, not by the shape of the cell.

Sticksare light-sensitive cells responding to lower light intensity than cones, but are unable to distinguish colors. They provide scotopic vision.

Photopic (day) vision
Color vision;
 * secured barrel cones ;
 * perceived brightness >10 2 cd/m 2 ;
 * fast adaptation to light (20–60 s);
 * maximum sensitivity for a wavelength of 555 nm;
 * perceived wavelengths of 400 to 750 nm.

Scotopic (night) vision

 * Perception of only brightness (changes in intensity);
 * secured by sticks only ;
 * perceived brightness 10 -3 cd/m 2 ;
 * two-phase adaptation, full adaptation after 40 to 60 minutes;
 * the maximum for wavelengths is around 500 nm.

Mesopic (twilight) vision

 * Brightness between the two previous values;
 * vision is provided by both rods and cones;
 * the spectral sensitivity of the eye is different from the spectral sensitivity in photopic vision.

Defects of the eye
For more information go to: Refractive defects of the eye.

Refractive defects of the eye are caused by poor properties of its refractive surfaces. Parallel rays entering the eye are not concentrated on the retina. Therefore, the image focus of the eye's optical system does not lie on it. This phenomenon may not be caused only by incorrect refraction of light rays. More often the problems are caused by axial defects of the eye - the refractive medium is fine, but the eye is of different length. Axial and refractive errors manifest themselves in the same way and can even be combined. In general, larger refractive errors cause fewer secondary problems. The image is hazy or blurred and the eye cannot correct this defect. The situation is different for smaller refractive errors. The eye is able to compensate for these to a certain extent, this effort leads to muscle and nerve exhaustion. This results in tiredness and watery eyes, headaches, and other symptoms. In an eye without a refractive error, the rays passing through the optical system intersect at the focus on the surface of the retina, such an eye is called emmetropic. We often encounter an ametropic eye, when the rays on the retina do not intersect. We distinguish between spherical and aspheric defects. Spherical defects are corrected with spherical lenses. These include myopia – Myopia and hypermetropia - Farsightedness. Myopia - the eye is physiologically too long. A sharp image is projected in front of the retina. The patient sees poorly at a distance, but well at near. Hypermetropia - the eye is physiologically too short. A sharp image would be projected behind the retina, the image is out of focus on the retina. We correct aspheric defects with aspheric, so-called toric, lenses. The most common aspheric disorder is astigmatism, where the patient sees unsharply both near and far. It causes asymmetry of the optical power of the cornea, or rarely of the lens.

The cornea has different curvatures in two mutually perpendicular axes.

Eye defects include presbyopia (farsightedness). Here, however, it is a natural process of loss of flexibility of the optical apparatus of the eye and thus a reduction of accommodation abilities in older age.

Correction of eye defects
For more information go to: Methods of correction of refractive errors.

Eye defects can be corrected with lenses (glasses, contact lenses) or laser surgery, the goal being to induce emmetropia. Dispersions are used to correct myopia. The ideal optical strength of the dispersion is such that the patient can see clearly to infinity (5-6 meters in practice). To correct hypermetropia, couplings are used, the optical power is chosen so that the patient can read the text at a distance of 25 cm. Astigmatism is corrected with cylindrical or toric glasses, in the case of irregular astigmatism, correction is difficult. For Presbyopia, the patient can use monofocal glasses for multiple distances or bifocal (and multifocal) glasses.

related articles

 * Defects of the eye
 * The principle of vision
 * The optical system of the eye
 * Eye (histology)
 * Biochemistry of the vision process
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