Under the action of light on the retina, chemical changes occur in the pigments located in the outer segments of the rods and cones. As a result a photo chemical reaction photoreceptors are stimulated retina.

Light-sensitive pigments were discovered in the retina of animals in the late 70s of the last century, and it was shown that these substances fade in the light. The retinal rods of humans and many animals contain the pigment rhodopsin, or visual purple, composition, properties and chemical transformations which have been studied in detail in recent decades (Wold et al.). The pigment iodopsin was found in the cones of birds. Apparently, there are also other light-sensitive pigments in the cones. Rushton indicates the presence of pigments in cones - chlorolab and erythrolab; the first of them absorbs the rays corresponding to the green, and the second - the red part of the spectrum.

Rhodopsin is a high molecular weight compound consisting of retinene - vitamin A aldehyde - and opsin protein. Under the action of light, a cycle of chemical transformations of this substance occurs. By absorbing light, retinene passes into its geometric isomer, characterized by the fact that its side chain is straightened, which leads to disruption of the bond of retinene with the protein. In this case, some intermediate substances are first formed - lumprodopsin and metarhodopsin, after which retinene is cleaved from opsin. Under the influence of an enzyme called retinene reductase, the latter is converted into vitamin A, which comes from the outer segments of the rods into the cells of the pigment layer.

When the eyes are darkened, regeneration of visual purple occurs, i.e., resynthesis of rhodopsin. This process requires that the retina receive the cis-isomer of vitamin A, from which retinene is formed. In the absence of vitamin A in the body, the formation of rhodopsin is sharply disrupted, which leads to the development of the above-mentioned night blindness. The formation of retinene from vitamin A is an oxidative process that occurs with the participation of the enzyme system. In the isolated retina of mammals, in which oxidative processes are disturbed, rhodopsin is not reduced.

A photo chemical processes in the retina occur very economically, i.e., under the action of even very bright light, only a small part of the rhodopsin present in the sticks is split. So, according to Wald, under the action of light with an intensity of 100 lux, after 5 seconds, only 1200 molecules of visual purple are split in each stick out of the 18 million molecules of this substance present in it, i.e., about 0.005% of rhodopsin decomposes.

The absorption of light by rhodopsin and its splitting are different depending on the wavelength of the light rays acting on it. Rhodopsin extracted from the human retina shows maximum absorption under the influence of light rays with a wavelength of about 500 mm k, which lie in the green part of the spectrum. It is these rays that seem the brightest in the dark. Comparison of the curve of absorption and discoloration of rhodopsin under the action of light of different wavelengths with the curve of the subjective assessment of the brightness of light in the dark reveals their complete coincidence ( rice. 215).

branch of chemistry that studies chemical reactions , occurring under the influence of light. Optics is closely related to optics (see optics) and optical radiation (see optical radiation). The first photochemical regularities were established in the 19th century. (see Grotgus law, Bunsen - Roscoe law (See Bunsen - Roscoe law)) . As an independent field of science, physics took shape in the first third of the 20th century, after the discovery Einstein's law, The molecule of matter, which has become the main one in F. When a light quantum is absorbed, the molecule of a substance passes from the ground state to an excited state, in which it enters into a chemical reaction. The products of this primary reaction (the actual photochemical one) are often involved in various secondary reactions (the so-called dark reactions) leading to the formation of final products. From this point of view, physics can be defined as the chemistry of excited molecules formed as a result of the absorption of light quanta. Often, a more or less significant part of the excited molecules does not enter into a photochemical reaction, but returns to the ground state as a result of various types of photophysical deactivation processes. In some cases, these processes can be accompanied by the emission of a quantum of light (fluorescence or phosphorescence). The ratio of the number of molecules involved in a photochemical reaction to the number of absorbed light quanta is called the quantum yield of the photochemical reaction. The quantum yield of the primary reaction cannot be greater than one; usually this value is much less than unity due to effective deactivation. As a result of dark reactions, the total quantum yield can be much greater than unity.

The most typical photochemical reaction in the gas phase is the dissociation of molecules with the formation of atoms and radicals. So, under the action of short-wave ultraviolet (UV) radiation, to which, for example, oxygen is exposed, the resulting excited O 2 molecules * dissociate into atoms:

O2 +hν O*2 , O*2 →O+O.

These atoms enter into a secondary reaction with O 2, forming ozone: O + O 2 → O 3.

Such processes occur, for example, in the upper layers of the atmosphere under the action of solar radiation (see Ozone in the atmosphere).

When a mixture of chlorine with saturated hydrocarbons (See Saturated hydrocarbons) (RH, where R is alkyl) is illuminated, the latter are chlorinated. The primary reaction is the dissociation of a chlorine molecule into atoms, followed by a chain reaction (See Chain reactions) of the formation of chlorine hydrocarbons:

Cl2+ hν →

Cl + RH → HCl + R

R + Cl 2 → RCl + Cl, etc.

The total quantum yield of this chain reaction much more than unity.

When a mixture of mercury vapor and hydrogen is illuminated with a mercury lamp, light is absorbed only by mercury atoms. The latter, passing into an excited state, cause the dissociation of hydrogen molecules:

Hg* + H 2 → Hg + H + H.

This is an example of a sensitized photochemical reaction. Under the action of a quantum of light, which has a sufficiently high energy, the molecules turn into ions. This process, called photoionization, is conveniently observed with a mass spectrometer.

The simplest photochemical process in the liquid phase is electron transfer, i.e., a light-induced redox reaction. For example, when UV light acts on an aqueous solution containing Fe 2 + , Cr 2 + , V 2 + ions, etc., an electron passes from an excited ion to a water molecule, for example:

(Fe 2 +) * + H 2 O → Fe 3 + + OH - + H +.

Secondary reactions lead to the formation of a hydrogen molecule. Electron transfer that can occur upon absorption visible light characteristic of many dyes. Phototransfer of an electron with the participation of a chlorophyll molecule is the primary act of Photosynthesis, a complex photobiological process that occurs in a green leaf under the action of sunlight.

In the liquid phase, molecules of organic compounds with multiple bonds and aromatic rings can participate in various dark reactions. In addition to breaking bonds, leading to the formation of radicals and diradicals (for example, carbenes (See Carbens)) , as well as heterolytic substitution reactions, numerous photochemical processes of isomerization are known (See Isomerization) , rearrangements, the formation of cycles, etc. There are organic compounds that isomerize under the action of UV light and acquire color, and when illuminated with visible light, they again turn into the original colorless compounds. This phenomenon is called photochromia. special case reversible photochemical transformations.

The task of studying the mechanism of photochemical reactions is very difficult. The absorption of a light quantum and the formation of an excited molecule occur over a time of about 10 - 15 sec. For organic molecules with multiple bonds and aromatic rings, which are of greatest interest to physics, there are two types of excited states, which differ in the magnitude of the total spin of the molecule. The latter can be equal to zero (in the ground state) or one. These states are called singlet and triplet states, respectively. The molecule passes into the singlet excited state directly upon absorption of a light quantum. The transition from the singlet to the triplet state occurs as a result of a photophysical process. The lifetime of a molecule in an excited singlet state is 10 -8 sec; in the triplet state - from 10 -5 -10 -4 sec(liquid media) up to 20 sec(hard media, such as solid polymers). Therefore, many organic molecules enter into chemical reactions precisely in the triplet state. For the same reason, the concentration of molecules in this state can become so significant that the molecules begin to absorb light, passing into a highly excited state, in which they enter into the so-called. two-quantum reactions. An excited A* molecule often forms a complex with an unexcited A molecule or with a B molecule. Such complexes, which exist only in an excited state, are called excimers (AA)* or exciplexes (AB)*, respectively. Exciplexes are often precursors to a primary chemical reaction. The primary products of a photochemical reaction - radicals, ions, radical ions and electrons - quickly enter into further dark reactions in a time that usually does not exceed 10 -3 sec.

One of the most effective methods studies of the mechanism of photochemical reactions - flash photolysis , the essence of which is to create a high concentration of excited molecules by illuminating the reaction mixture with a short but powerful flash of light. The short-lived particles that arise in this case (more precisely, the excited states and the primary products of the photochemical reaction named above) are detected by their absorption of the "probing" beam. This absorption and its change in time is recorded using a photomultiplier and an oscilloscope. This method can be used to determine both the absorption spectrum of an intermediate particle (and thereby identify this particle) and the kinetics of its formation and disappearance. In this case, laser pulses with a duration of 10 -8 sec and even 10 -11 -10 -12 sec, which makes it possible to study the earliest stages of the photochemical process.

The field of practical application of F. is extensive. Methods of chemical synthesis based on photochemical reactions are being developed (see Photochemical reactor, Solar photosynthetic installation) . Found application, in particular for recording information, photochromic compounds. With the use of photochemical processes, relief images are obtained for microelectronics (See Microelectronics) , printing forms for printing (see also Photolithography). Of practical importance is photochemical chlorination (mainly of saturated hydrocarbons). The most important field of practical application of photography is photography. In addition to the photographic process based on the photochemical decomposition of silver halides (mainly AgBr), various non-silver photography techniques are becoming increasingly important; for example, photochemical decomposition of diazo compounds (See Diazo compounds) underlies diazotyping (See. Diazotyping).

Lit.: Turro N. D., Molecular photochemistry, trans. from English, M., 1967; Terenin A. N., Photonics of molecules of dyes and related organic compounds, L., 1967; Calvert D. D., Pitts D. N., Photochemistry, trans. from English, M., 1968; Bagdasaryan Kh. S., Two-quantum photochemistry, M., 1976.

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  Encyclopedic Dictionary of Nanotechnology

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The retinal rods of humans and many animals contain pigment rhodopsin, or visual purple, the composition, properties and chemical transformations of which have been studied in detail in recent decades. Pigment found in cones iodopsin. The cones also contain the pigments chlorolab and erythrolab; the first of them absorbs the rays corresponding to the green, and the second - the red part of the spectrum.

Rhodopsin is a high-molecular compound (molecular weight 270,000), consisting of retinal - vitamin A aldehyde and opsin protein. Under the action of a light quantum, a cycle of photophysical and photochemical transformations of this substance occurs: retinal isomerizes, its side chain is straightened, the bond between retinal and protein is broken, and the enzymatic centers of the protein molecule are activated. The retinal is then cleaved from the opsin. Under the influence of an enzyme called retinal reductase, the latter is converted into vitamin A.

When the eyes are darkened, the regeneration of visual purple occurs, i.e. resynthesis of rhodopsin. This process requires that the retina receives the cis-isomer of vitamin A, from which retinal is formed. If vitamin A is absent in the body, the formation of rhodopsin is sharply disrupted, which leads to the development of the above-mentioned night blindness.

Photochemical processes in the retina occur very sparingly; under the action of even very bright light, only a small part of the rhodopsin present in the sticks is split.

The structure of iodopsin is close to that of rhodopsin. Iodopsin is also a compound of retinal with the protein opsin, which is produced in cones and is different from rod opsin.

The absorption of light by rhodopsin and iodopsin is different. Iodopsip absorbs yellow light with a wavelength of about 560 nm to the greatest extent.

color vision

On the long-wave edge of the visible spectrum are red rays (wavelength 723-647 nm), on the short-wavelength - violet (wavelength 424-397 nm). Mixing rays of all spectral colors gives white. White color can also be obtained by mixing two so-called paired complementary colors: red and blue, yellow and blue. If you mix colors taken from different pairs, you can get intermediate colors. As a result of mixing the three primary colors of the spectrum - red, green and blue - any color can be obtained.

Theories of color vision. There are a number of theories of color perception; The three-component theory enjoys the greatest recognition. It asserts the existence in the retina of three different types color-perceiving photoreceptors - cones.

The existence of a three-component mechanism for the perception of colors was also mentioned by M.V. Lomonosov. This theory was later formulated in 1801. T. Young and then developed G. Helmholtz. According to this theory, cones contain various photosensitive substances. Some cones contain a substance that is sensitive to red, others to green, and still others to violet. Every color has an effect on all three color-sensing elements, but to varying degrees. These excitations are summed up by visual neurons and, having reached the cortex, give the sensation of one color or another.

According to another theory proposed E. Goering, there are three hypothetical photosensitive substances in the cones of the retina: 1) white-black, 2) red-green, and 3) yellow-blue. The breakdown of these substances under the influence of light leads to a sensation of white, red or yellow. Other light rays cause the synthesis of these hypothetical substances, resulting in a sensation of black, green and blue.

The most compelling confirmation in electrophysiological studies was received by the three-component theory of color vision. In experiments on animals, microelectrodes were used to divert impulses from single ganglion cells of the retina when it was illuminated with different monochromatic beams. It turned out that electrical activity in most neurons arose under the action of rays of any wavelength in the visible part of the spectrum. Such elements of the retina are called dominators. In other ganglion cells (modulators), impulses arose only when illuminated by rays of only a certain wavelength. 7 modulators have been identified that respond optimally to light with different wavelengths (from 400 to 600 nm.). R. Granit believes that the three components of color perception, proposed by T. Jung and G. Helmholtz, are obtained by averaging the spectral sensitivity curves of modulators, which can be grouped according to the three main parts of the spectrum: blue-violet, green and orange.

When measuring the absorption of rays of different wavelengths by a single cone with a microspectrophotometer, it turned out that some cones maximally absorb red-orange rays, others - green, and still others - blue rays. Thus, three groups of cones have been identified in the retina, each of which perceives rays corresponding to one of the primary colors of the spectrum.

The three-component theory of color vision explains a number of psychophysiological phenomena, such as sequential color images, and some facts of the pathology of color perception (blindness in relation to individual colors). AT last years many so-called opponent neurons have been studied in the retina and visual centers. They differ in that the action of radiation on the eye in some part of the spectrum excites them, and in other parts of the spectrum it inhibits them. It is believed that such neurons most effectively encode color information.

color blindness. Color blindness occurs in 8% of men, its occurrence is due to the genetic absence of certain genes in the sex-determining unpaired X chromosome in men. In order to diagnose color blindness, the subject is offered a series of polychromatic tables or allowed to select the same objects of different colors by color. Diagnosis of color blindness is important in professional selection. People with color blindness cannot be drivers of transport, as they cannot distinguish the colors of traffic lights.

There are three types of partial color blindness: protanopia, deuteranopia, and tritanopia. Each of them is characterized by the absence of perception of one of the three primary colors. People suffering from protanopia ("red-blind") do not perceive red, blue-blue rays seem colorless to them. Persons suffering from deuteranopia ("green-blind") do not distinguish green from dark red and blue. With tritanopia, a rare anomaly of color vision, rays of blue and violet are not perceived.

Accommodation

For a clear vision of an object, it is necessary that the rays from its points fall on the surface of the retina, i.e. were focused here. When a person looks at distant objects, their image is focused on the retina and they are seen clearly. At the same time, close objects are not clearly visible, their image on the retina is blurry, since the rays from them are collected behind the retina. It is impossible to see objects equally clearly at different distances from the eye at the same time. It is easy to see this: when you look from near to distant objects, you stop seeing it clearly.

The adaptation of the eye to clearly see objects at different distances is called accommodation . During accommodation there is a change in the curvature of the lens and, consequently, its refractive power. When viewing close objects, the lens becomes more convex, due to which the rays diverging from the luminous point converge on the retina. The mechanism of accommodation is reduced to the contraction of the ciliary muscles, which change the convexity of the lens. The lens is enclosed in a thin transparent capsule, passing along the edges into the fibers of the zinn ligament attached to the ciliary body. These fibers are always taut and stretch the capsule, which compresses and flattens the lens. The ciliary body contains smooth muscle fibers. With their contraction, the traction of the zinn ligaments is weakened, which means that the pressure on the lens decreases, which, due to its elasticity, takes on a more convex shape. Thus, the ciliary muscles are accommodative muscles. They are innervated by parasympathetic fibers of the oculomotor nerve. The introduction of atropine into the eye causes a violation of the transmission of excitation to this muscle, and, therefore, limits the accommodation of the eyes when considering close objects. On the contrary, parasympathomimetic substances - pilocarpine and ezerin - cause contraction of this muscle.

Presbyopia. The lens becomes less elastic with age, and when the tension of the zinn ligaments is weakened, its convexity either does not change, or increases only slightly. Therefore, the nearest point of clear vision moves away from the eyes. This state is called senile farsightedness or presbyopia.

- Anatomy of vision

Anatomy of vision

vision phenomenon

When scientists explain vision phenomenon , they often compare the eye to a camera. Light, just as it happens with the lenses of the apparatus, enters the eye through a small hole - the pupil, located in the center of the iris. The pupil can be wider or narrower: in this way, the amount of light entering is regulated. Further, the light is directed to the back wall of the eye - the retina, as a result of which a certain picture (image, image) appears in the brain. Similarly, when light hits the back of a camera, the image is captured on film.

Let's take a closer look at how our vision works.

First, the visible parts of the eye, to which they belong, receive light. Iris("input") and sclera(white of the eye). After passing through the pupil, the light enters the focusing lens ( lens) of the human eye. Under the influence of light, the pupil of the eye constricts without any effort or control of the person. This is because one of the muscles of the iris - sphincter- sensitive to light and reacts to it by expanding. The constriction of the pupil occurs due to the automatic control of our brain. Modern self-focusing photographic cameras do much the same thing: a photoelectric "eye" adjusts the diameter of the entrance hole behind the lens, thus metering the amount of incoming light.

Now let's turn to the space behind the eye lens, where the lens is located, a vitreous gelatinous substance ( vitreous body) and finally - retina, an organ that is truly admired for its structure. The retina covers the vast surface of the fundus. It is a unique organ with a complex structure unlike any other body structure. The retina of the eye consists of hundreds of millions of light-sensitive cells called "rods" and "cones". unfocused light. sticks are designed to see in the dark, and when they are activated, we can perceive the invisible. Film can't do that. If you use film designed for shooting in dim light, it will not be able to capture a picture that is visible in bright light. But the human eye has only one retina, and it is able to operate in different conditions. Perhaps it can be called a multifunctional film. cones, unlike sticks, work best in the light. They need light to provide sharp focus and clear vision. The highest concentration of cones is in the area of ​​the retina called the macula ("spot"). In the central part of this spot is the fovea centralis (eye fossa, or fovea): it is this area that makes the most acute vision possible.

The cornea, pupil, lens, vitreous body, as well as the size of the eyeball - all this depends on the focusing of light as it passes through certain structures. The process of changing the focus of light is called refraction (refraction). Light that is more accurately focused hits the fovea, while less focused light scatters on the retina.

Our eyes are capable of distinguishing about ten million gradations of light intensity and about seven million shades of colors.

However, the anatomy of vision is not limited to this. Man, in order to see, uses both his eyes and his brain at the same time, and for this a simple analogy with a camera is not enough. Every second, the eye sends about a billion pieces of information to the brain (more than 75 percent of all the information we perceive). These portions of light turn in consciousness into amazingly complex images that you recognize. Light, taking the form of these recognizable images, appears as a kind of stimulant for your memories of the events of the past. In this sense, vision acts only as a passive perception.

Almost everything we see is what we have learned to see. After all, we come into life without having any idea how to extract information from the light falling on the retina. In infancy, what we see means nothing or almost nothing to us. Impulses stimulated by light from the retina enter the brain, but for the baby they are only sensations, devoid of meaning. As a person grows up and learns, he begins to interpret these sensations, tries to understand them, to understand what they mean.