Photochemical processes in the retina consist in the fact that the visual purple (rhodopsin) located in the outer segments of the rods is destroyed under the influence of light and restored in the dark. Recently, Rushton (1967) and Weale (1962) have been studying the role of visual purple in the process of the action of light on the eye.
The devices they constructed make it possible to measure the thickness of the rhodopsin layer in the retina of a living human eye, which disintegrates under the influence of light. The results of the studies allowed the authors to conclude that there is no direct relationship between changes in light sensitivity and the amount of disintegrated visual purple.
This may indicate more complex processes occurring in the retina when exposed to visible radiation or, as it seems to us, an imperfect method (use of atropine, use of an artificial pupil, etc.).
The action of light is not explained solely by a photochemical reaction. It is generally accepted that when light hits the retina, action currents arise in the optic nerve, which are recorded by the higher centers of the cerebral cortex.
When recording action currents over time, a retinogram is obtained. As analysis of the electroretinogram shows, it is characterized by an initial latent period (the time from the moment of exposure to the light flux until the appearance of the first impulses), a maximum (an increase in the number of impulses) and a smooth decrease with a preliminary slight increase (the latent period of the final effect).
So, at the same brightness of the stimulus, the frequency of impulses depends on the nature of the preliminary adaptation of the eye; if the eye was adapted to light, then it decreases, and if adapted to darkness, it increases.
In addition to reacting to light, the visual analyzer carries out certain visual work. However, in all likelihood, the mechanisms involved in the process of light perception and the details of the object when performing visual work will not be completely identical.
If the analyzer responds to fluctuations in the level of light flux by increasing or decreasing the area of the receptive fields of the retina, then to the complication of the object of perception - by changing the optical system of the eye (convergence, accommodation, papillomotor reaction, etc.).
Visible radiation affects various functions of the visual analyzer: on light sensitivity and adaptation, contrast sensitivity and visual acuity, stability of clear vision and speed of discrimination, etc.
“Clinic of diseases, physiology and hygiene in adolescence”, G.N. Serdyukovskaya
The muscles of the pupil, having received the D signal, stop responding to the G signal, which is reported by the E signal. From this moment on, the pupil takes all possible part in enhancing the clarity of the image of an object on the retina, but the main role in this process belongs to the lens. In turn, the “center for regulating the strength of the retinal stimulus”, having received the signal E, transmits information to other centers, in ...
E. S. Avetisov considers the progression of myopia as a consequence of “overregulation,” when the “expedient” process of adapting an eye with a weakened accommodative ability to work at close range turns into its opposite. From the above, it becomes clear how important sufficient rational lighting is for the performance of the eye. It is of particular importance for teenagers who combine work and study. However, currently...
Luminous intensity and surface illumination are related by the following equality: I=EH2; E=I/H2; E=I*cos a/H2. where E is the surface illumination in lux; H - height of installation of the lamp above the illuminated surface in meters; I - luminous intensity in candles; a is the angle between the direction of light intensity and the axis of the lamp. Brightness (B) is the strength of light reflected from a surface in the direction...
Artificial lighting The following characteristics are taken as the basis for standardization, which determine the degree of tension in visual work. The accuracy of visual work, characterized by the smallest size of the part being considered. The term “part” in the standards does not mean the product being processed, but an “object” that has to be examined during the work process, for example, a thread of fabric, a scratch on the surface of the product, etc. The degree of lightness of the background against which the object is viewed….
Reducing the illumination by one level is allowed for industrial premises with short-term occupancy of people, as well as in premises where there is equipment that does not require constant maintenance. When installing combined lighting on a work surface, the illumination from general lighting fixtures should be at least 10% of the combined lighting standards, but for teenagers, obviously, it should be at least 300 lux...
The phenomenon of luminescence has long been known - a substance absorbs light of a certain frequency, and itself creates scattered radiation of a different frequency. Back in the 19th century, Stokes established the rule - the frequency of scattered light is less than the frequency of absorbed light (ν absorb > ν dis); the phenomenon occurs only when sufficiently high frequency of incident light.
In some cases, luminescence occurs almost inertia-free - it appears immediately and stops 10 -7 -10 -8 s after the cessation of illumination. This special case of luminescence is sometimes called fluorescence. But a number of substances (phosphorus and others) have a long afterglow, lasting (gradually weakening) minutes and even hours. This type of luminescence is called phosphorescence. When heated, the body loses its ability to phosphoresce, but retains the ability to luminesce.
Multiplying both sides of the inequality expressing Stokes' rule by Planck's constant, we obtain:
Consequently, the energy of a photon absorbed by an atom is greater than the energy of the photon emitted by it; Thus, the photonic nature of light absorption processes is manifested here too.
We will consider the existing deviations from the Stokes rule later (§ 10.6).
In the phenomena of photochemistry - chemical reactions under the influence of light - it was also possible to establish the existence of the lowest frequency required for a reaction to occur. This is quite understandable from a photonic point of view: for a reaction to occur, the molecule must receive sufficient additional energy. Often the phenomenon is masked by additional effects. Thus, it is known that a mixture of hydrogen H 2 with chlorine Cl 2 exists in the dark for a long time. But even under weak illumination with light of a sufficiently high frequency, the mixture explodes very quickly.
The reason lies in the occurrence of secondary reactions. A hydrogen molecule, having absorbed a photon, can dissociate (main reaction):
H 2 +hν -> H + H.
Since atomic hydrogen is much more active than molecular hydrogen, a secondary reaction occurs with the release of heat:
H+Cl 2 =HCl+Cl.
Thus, the H and Cl atoms are released. They interact with C1 2 and H 2 molecules, and the reaction grows very rapidly, once excited by the absorption of a small number of photons.
Among the various photochemical reactions, the reactions that occur during the photography process deserve attention. The camera creates a real (usually reduced) image on a layer of photographic emulsion containing silver bromide, which is capable of photochemical reactions. The number of reacted molecules is approximately proportional to the intensity of the light and the time of its action (exposure time when photographing). However, this number is relatively very small; the resulting “latent image” is subjected to a development process, when, under the influence of appropriate chemical reagents, additional release of silver bromide occurs at the centers generated during the photochemical reaction. Then follows the process of fixing (fixing) the image: unreacted light-sensitive silver bromide is transferred into solution and metallic silver remains on the photo layer, which determines the transparency of individual areas of the resulting negative Image (the more light absorbed, the darker the corresponding area). By illuminating the photographic paper (or film) through a negative, one obtains on the paper (after it has been developed and fixed) an illumination distribution corresponding to the object being photographed (of course, if the proper conditions for shooting and processing the photographic material are met). In color photography, the film contains three layers that are sensitive to three different parts of the spectrum.
These layers serve as light filters for each other, and the illumination of each of them is determined only by a certain part of the spectrum. Being much more complex than the black and white photographic process, the process of color photography is in principle no different from the first and is a typical photonic process.
“Methodological development of a section of the program” - Compliance of educational technologies and methods with the goals and content of the program. The social and pedagogical significance of the presented results of applying the methodological development. Diagnostics of planned educational results. - Cognitive - transformative - general educational - self-organizing.
“Modular educational program” - Requirements for module development. At German universities, the training module consists of three levels of disciplines. Module structure. Level 2 training courses are included in the module on a different basis. The content for an individual component is consistent with the content of the other components of the module.
“Organization of the educational process at school” - You won’t understand. Zzzz! (direct sound and sight according to the text). Application. A set of preventive exercises for the upper respiratory tract. RUN ON YOUR TOES Goal: development of auditory attention, coordination and sense of rhythm. Y-ah-ah! Objectives of physical education minutes. Criteria for assessing the health-saving component in a teacher’s work.
“Summer holiday” - Musical relaxation, health tea. Monitoring the regulatory framework of the subjects of the summer health campaign. Section 2. Work with personnel. Continued dance studies and practical classes. Development of recommendations based on the results of the past stages. Expected results. Stages of program execution.
“School of Social Success” - New formula of standards - requirements: Primary education. Tr - to the results of mastering basic educational programs. Organizational section. Popova E.I. Introduction of the Federal State Educational Standard of the NOO. Subject results. Target section. 2. Basic Educational Program. 5. Materials of the methodological meeting.
"KSE" - Basic concepts of a systems approach. Concepts of modern natural science (CSE). Science as critical knowledge. - Whole - part - system - structure - element - set - connection - relationship - level. The concept of "concept". Humanities Psychology Sociology Linguistics Ethics Aesthetics. Physics Chemistry Biology Geology Geography.
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branch of chemistry in which chemical reactions are studied , occurring under the influence of light. Physics is closely related to optics (See Optics) and optical radiation (See Optical radiation). The first photochemical laws were established in the 19th century. (see Grotthus law, Bunsen - Roscoe law (See Bunsen - Roscoe law)) . Physics took shape as an independent field of science in the first third of the 20th century, after Einstein’s discovery of the law , which has become the basic one in Ph. When a molecule of a substance is absorbed by a quantum of light, it passes from the ground state to an excited state, in which it enters into a chemical reaction. The products of this primary reaction (actually photochemical) often participate in various secondary reactions (so-called dark reactions), leading to the formation of final products. From this point of view, phosphorus can be defined as the chemistry of excited molecules formed by the absorption of light quanta. Often, a more or less significant part of excited molecules does not enter into a photochemical reaction, but returns to the ground state as a result of various kinds of photophysical deactivation processes. In some cases, these processes may be accompanied by the emission of a light quantum (fluorescence or phosphorescence). The ratio of the number of molecules entering 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 unity; this value is usually significantly less than unity due to efficient decontamination. As a result of dark reactions, the total quantum yield can be significantly greater than unity.
The most typical photochemical reaction in the gas phase is the dissociation of molecules with the formation of atoms and radicals. Thus, under the action of short-wave ultraviolet (UV) radiation, to which, for example, oxygen is exposed, excited O 2 molecules are formed * 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 influence of solar radiation (see Ozone in the atmosphere).
When a mixture of chlorine and saturated hydrocarbons (See Saturated hydrocarbons) (RH, where R is alkyl) is illuminated, the latter are chlorinated. The primary reaction is the dissociation of the 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 is significantly greater than unity.
When a mercury lamp illuminates a mixture of mercury vapor and hydrogen, the 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 influence of a quantum of light with sufficiently high energy, molecules turn into ions. This process, called photoionization, can be conveniently observed using a mass spectrometer.
The simplest photochemical process in the liquid phase is electron transfer, i.e., a light-induced redox reaction. For example, when exposed to UV light on an aqueous solution containing Fe 2 + , Cr 2 + , V 2 + ions, etc., an electron passes from the 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, which can occur upon absorption of visible light, is characteristic of many dyes. Electron phototransfer 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 influence of sunlight.
In the liquid phase, molecules of organic compounds with multiple bonds and aromatic rings can participate in a variety of dark reactions. In addition to the cleavage of bonds leading to the formation of radicals and biradicals (for example, carbenes (See Carbenes)) , as well as heterolytic substitution reactions, numerous photochemical isomerization processes are known (See Isomerization) , rearrangements, formation of cycles, etc. There are organic compounds that, under the influence of UV light, isomerize and acquire color, and when illuminated with visible light they again transform into the original colorless compounds. This phenomenon, called photochromia, is a special case of reversible photochemical transformations.
The task of studying the mechanism of photochemical reactions is very complex. The absorption of a light quantum and the formation of an excited molecule occur in 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 that differ in the value 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, respectively. The molecule goes into a 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 Photochemistry 10 -8 sec; in the triplet state – from 10 -5 –10 -4 sec(liquid media) up to 20 sec(hard media, for example solid polymers). Therefore, many organic molecules enter into chemical reactions 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, moving into a highly excited state, in which they enter the so-called. two-quantum reactions. An excited molecule A* often forms a complex with an unexcited molecule A or with a molecule B. Such complexes, which exist only in an excited state, are called excimers (AA)* or exciplexes (AB)*, respectively. Exciplexes are often precursors to the 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 usually not exceeding 10 -3 sec.
One of the most effective methods for studying the mechanism of photochemical reactions is pulsed 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 above-mentioned primary products of the photochemical reaction) are detected by their absorption of the “probing” beam. This absorption and its change over time is recorded using a photomultiplier tube and an oscilloscope. Using this method, it is possible 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 plant) . Photochromic compounds have found application, in particular for recording information. Using photochemical processes, relief images are obtained for microelectronics (See Microelectronics) , printing forms for printing (see also Photolithography). Photochemical chlorination (mainly of saturated hydrocarbons) is of practical importance. The most important area of practical application of photography is photography. In addition to the photographic process based on the photochemical decomposition of silver halides (mainly AgBr), various methods of non-silver photography are becoming increasingly important; for example, photochemical decomposition of diazo compounds (See Diazo compounds) underlies diazotype (See Diazotype).
Lit.: Turro N.D., Molecular photochemistry, trans. from English, M., 1967; Terenin A. N., Photonics of molecules of dyes and related organic compounds, Leningrad, 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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