Lesson objectives: formation of ideas about the structure of the eye and the mechanisms of operation of the optical system of the eye; elucidation of the conditionality of the structure of the optical system of the eye by the laws of physics; developing the ability to analyze the studied phenomena; the formation of a respectful attitude towards one's own health and the health of others.
Equipment: table "Organ of vision", model "Human eye"; light collecting lens, large curvature lens, small curvature lens, light source, task cards; on students' desks: light-collecting lens, light-scattering lens, screen with a slot, light source, screen.
DURING THE CLASSES
Biology teacher. A person has a system of orientation in the world around him - sensory system, which helps not only to navigate, but also to adapt to changing environmental conditions. In the previous lesson, you began to get acquainted with the structure of the organ of vision. Let's remember this material. To do this, you must complete the task on the card and answer the questions.
Review questions
- Why does a person need sight?
- Which body performs this function?
- Where is the eye located?
- Name the membranes of the eye and their functions.
- Name the parts of the eye that protect it from damage.
There is a table on the board " Organ of vision", On the teacher's table - the model" Human eye ". After collecting the students' answer cards, the biology teacher checks their completion by naming and showing parts of the eye on the model and poster with the students.
Students are given a second card.
Biology teacher. Based on knowledge anatomical structure eyes, name which parts of the eye can perform an optical function.
(Students, referring to the model of the eye, come to the conclusion that the optical system of the eye consists of the cornea, lens, vitreous body and retina.)
Physics teacher. What optical device does the lens remind you of?
Students. Biconvex lens.
Physics teacher. What types of lenses do you still know and what are their properties?
Students. A biconvex lens is a converging lens, i.e. rays passing through the lens are collected at one point called the focus. A biconcave lens is a scattering lens, the rays passing through the lens are scattered in such a way that the continuation of the rays is collected in an imaginary focus.
(Physics teacher draws(rice. 1) on the blackboard, and the students in the notebook the path of the rays in the collecting and diffusing lens.)
Rice. 1. The path of rays in the collecting and diffusing lenses (F - focus)
Physics teacher. What will be the image if the subject is behind twice the focal length of the converging lens?
(Students draw the path of rays in notebooks in this case (Fig. 2) and make sure that the image is reduced, real, inverted.)
Rice. 2. Construction of the image in the collecting lens
Frontal experiment
On each table, students have a collecting and diffusing lens, a current source, an electric lamp on a stand, a screen with a slot in the shape of the letter G, and a screen.
The physics teacher invites students to choose a biconvex, i.e. collecting lens and make sure experimentally that the collecting lens gives an inverted image. Students assemble the installation (Fig. 3) and, by moving the lens relative to the screen, achieve a clear image of an inverted letter G.
(Students are convinced by experience that the image is actually inverted and appears clearly on the screen only when the screen is positioned relative to the lens..)
Rice. 3. Installation diagram for demonstrating the path of rays in a collecting lens
Biology teacher. Since the lens, cornea and vitreous are a collecting lens, the optical system of the eye gives an inverted reduced image, and we should see the world upside down. What allows you to see objects upside down?
Students. Normal, and not upside down, vision of objects is due to their repeated "overturning" in the cortical region visual analyzer.
Biology teacher. We see objects well at different distances. This is due to the muscles that attach to the lens and, by contracting, regulate its curvature.
Physics teacher. Let us consider experimentally how the properties of a lens change depending on its curvature. The smaller the radius of curvature, the smaller the focal length - such lenses are called short-focus lenses, lenses with a small curvature, i.e. with a large radius of curvature are called long-focus (Fig. 4).
Rice. 4. Changing the properties of the lens depending on its curvature
Biology teacher. When looking at closely spaced objects, the lens decreases the radius of curvature, and it acts as a short-focus lens. When looking at distant objects, the lens increases the radius of curvature, and it acts as a long-focus lens. In both cases, this is necessary so that the image is always focused on the retina. The ability to clearly see objects distant at different distances, due to a change in the curvature of the lens, is called accommodation (students write the definition in a notebook).
There are deviations in the structure of the eye or in the work of the lens.
With myopia, the image is focused in front of the retina due to excessive curvature of the lens or lengthening of the axis of the eye. With farsightedness, the image is focused behind the retina due to insufficient curvature of the lens or the shortened axis of the eye.
Physics teacher. What lenses are needed to correct myopia, and which ones to correct hyperopia?
Students. Myopia is a diffusing lens, hyperopia is a collecting lens.
(Physics teacher demonstrating experience experimentally proves the validity of students' conclusions.)
Biology teacher. There is another deviation from the norm in the optical system of the human eye - this is astigmatism. Astigmatism is the impossibility of convergence of all rays at one point, in one focus. This is due to deviations from the spherical curvature of the cornea. Cylindrical lenses are used to correct astigmatism.
conclusions
Students, together with a biology teacher, formulate the basic rules of vision hygiene:
- protect eyes from mechanical stress;
- read in a well-lit room;
- keep the book at a certain distance (33–35 cm) from the eyes;
- the light should fall from the left;
- you cannot lean close to the book, because this can lead to the development of myopia;
- you cannot read in a moving vehicle, because due to the instability of the position of the book, the focal length changes all the time, which leads to a change in the curvature of the lens, a decrease in its elasticity, as a result of which the ciliary muscle weakens and vision is impaired.
thanks
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What is hyperopia?
Farsightedness Is an eye disease characterized by damage to its refractive system, as a result of which images of close objects are focused not on the retina ( as normal), and behind it. With farsightedness, people see the outlines of objects as indistinct, vague, and the closer the object is to the eye, the worse it is recognized by a person. In order to understand the causes, mechanisms of development and principles of treatment of hyperopia, certain knowledge about the structure and functioning of the eye is required.
Conventionally, two sections are distinguished in the human eye - the retina and the refractive system of the eye. The retina is peripheral department visual analyzer, consisting of many light-sensitive nerve cells. Photons ( light particles), reflected from various surrounding objects, fall on the retina. As a result, nerve impulses are generated in photosensitive cells, which are sent to a special section of the cerebral cortex, where they are perceived as images.
The refractive system of the eye includes a complex of organs responsible for focusing images on the retina.
The refractive system of the eye includes:
- Cornea. This is the front, convex part eyeball, shaped like a hemisphere. The cornea has a constant refractive power of about 40 diopters ( diopter - a unit of measurement that determines the degree of refractive power of a lens).
- The lens. It is located behind the cornea and is a biconvex lens that is fixed by several ligaments and muscles. If necessary, the lens can change its shape, as a result of which its refractive power can also vary from 19 to 33 diopters.
- Watery moisture. It is a liquid located in special chambers of the eye in front of and behind the lens. It performs a nutritional function ( transports nutrients to the lens, cornea and other tissues) and protective function (contains immunoglobulins that can fight foreign viruses, bacteria and other microorganisms). Refractive power aqueous humor insignificant.
- Vitreous humor. A clear, jelly-like substance that fills the space between the lens and the retina. Refractive power vitreous is also negligible. Its main function is to maintain the correct shape of the eye.
The essence of farsightedness lies in the fact that the light beams passing through the refractive system of the eye are focused not directly on the retina, but behind it, as a result of which the image of the observed object turns out to be indistinct and blurry.
The reasons for the development of hyperopia
The cause of hyperopia can be both damage to the refractive structures of the eye, and the irregular shape of the eyeball itself. Depending on the cause and mechanism of development, there are:
- physiological hyperopia in children;
- congenital hyperopia;
- acquired hyperopia;
- age-related hyperopia ( presbyopia).
Physiological hyperopia in children
The structure of the eye in a newborn is different from that of an adult. In particular, the child has a more rounded shape of the eyeball, less pronounced curvature of the cornea and the refractive power of the lens. As a result of these features, the image in baby eye is projected not directly onto the retina, but behind it, which leads to hyperopia.Almost all newborns have a physiological hyperopia of about 4 - 5 diopters. As the child grows, the structure of his eye undergoes a number of changes, in particular, the anteroposterior axis of the eyeball lengthens, the curvature increases ( and refractive power) of the cornea and lens. All this leads to the fact that at the age of 7 - 8 years, the degree of hyperopia is only 1.5 - 2 diopters, and by the age of 14 ( when the formation of the eyeball is completely completed) in most adolescents, vision becomes completely normal.
Congenital hyperopia
Diagnose congenital ( pathological) farsightedness is possible only in children over 5 - 6 years old, since until this age the eyeball itself and the refractive structures of the eye continue to develop. At the same time, if a child aged 2 to 3 years has hyperopia of 5 to 6 diopters or more, it is highly likely that this phenomenon will not go away on its own as you grow up.The cause of congenital farsightedness can be various anomalies of the eyeball or the refractive system of the eye.
Congenital hyperopia may result from:
- Developmental disorders of the eyeball. If the eyeball is underdeveloped ( too small) or if its form is initially violated, then ( as the child grows) it may also develop incorrectly, as a result of which the child's farsightedness will not disappear, but may even progress.
- Corneal developmental disorders. As mentioned earlier, as a child grows up, the refractive power of his cornea increases. If this does not happen, the child's farsightedness will remain. Also more pronounced hyperopia ( more than 5 diopters) can be observed in children with congenital malformations of the cornea ( that is, if the cornea is initially too flat and its refractive power is extremely low).
- Lens developmental disorders. V this group include congenital lens displacement ( when it is not in its usual place), microfakia ( too small lens) and aphakia ( congenital absence of the lens).
Acquired farsightedness
Acquired hyperopia can develop as a result of damage to the refractive system of the eye ( cornea or lens), and also be a consequence of a decrease in the anteroposterior size of the eyeball. This may be due to eye injuries not carried out correctly. surgical operations, tumors in the orbit area ( during growth, they can squeeze the eyeball, changing its shape). Also, the cause of hyperopia can be acquired aphakia, in which the lens is removed due to various diseases, for example, after an eye injury with damage to the lens, with the development of cataracts ( lens opacity) etc.Age-related hyperopia ( presbyopia)
A separate form of acquired hyperopia is age-related ( senile) hyperopia. The reason for the development of this pathology is a violation of the structure and function of the lens, associated with the peculiarities of its development.The normal lens is a biconvex lens that sits behind the cornea. The lens substance itself is transparent, does not contain blood vessels and is surrounded by a lens capsule. Special ligaments are attached to this capsule, which hold the lens in suspension directly behind the cornea. These ligaments, in turn, are connected to the ciliary muscle, which regulates the refractive power of the lens. When a person looks into the distance, the fibers of the ciliary muscle relax. This contributes to the tension of the ligaments of the lens, as a result of which it itself flattens ( shrinks). As a result, the refractive power of the lens decreases and a person can focus his vision on objects far away. When examining objects close up, the opposite process takes place - the tension of the ciliary muscle leads to relaxation ligamentous apparatus lens, as a result of which it becomes more convex, and its refractive power increases.
An important feature of the lens is its continuous growth ( the diameter of the lens of a newborn is 6.5 mm, and that of an adult is 9 mm). The growth process of the lens is due to special cells located in the area of its edges. These cells have the ability to divide, that is, multiply. After division, the newly formed cell turns into a transparent lens fiber. New fibers begin to move towards the center of the lens, displacing older fibers, resulting in a denser substance called the lens nucleus in the central zone.
The described process underlies the development of presbyopia ( senile hyperopia). By about 40 years of age, the forming nucleus becomes so dense that it disrupts the elasticity of the lens itself. In this case, when the lens ligaments are strained, it itself flattens only partially, which is due to the dense nucleus located in its center. By the age of 60, the nucleus is sclerosed, that is, it reaches its maximum density.
It should be noted that the process of development of age-related hyperopia begins with early childhood, however, it becomes clinically noticeable only by the age of 40, which is manifested by a weakening of accommodation. It has been calculated that as a result of the formation and compaction of the lens nucleus, its accommodative ability decreases by about 0.001 diopters daily from the moment of birth until the age of 60.
Symptoms, signs and diagnosis of hyperopia
With congenital ( not physiological) farsightedness, the child may not make any complaints for a long time. This is due to the fact that from the moment of birth he sees closely spaced objects vaguely and does not know that this is not normal. In this case, parents can suspect hyperopia, based on the characteristic behavior of the child ( the child does not distinguish closely spaced objects, when reading, he moves the book far from his eyes, and so on).In the case of acquired hyperopia, the symptoms of the disease develop gradually, which is most characteristic of age-related hyperopia. The main complaint of such patients is the inability to clearly see nearby objects. This condition is aggravated by poor lighting, as well as when trying to read small text. At the same time, patients see more distant objects better, and therefore, when reading, they often push the book aside at arm's length ( the need to do this on a regular basis annoys many patients, which they mention in a conversation with a doctor).
Others characteristic manifestation farsightedness is asthenopia, that is, visual discomfort that occurs in patients while reading or working with small details. The development of this symptom is associated with a violation of accommodation. Normally, during reading, the refractive power of the lens increases slightly, which allows you to focus your gaze on a closely spaced text. However, people with farsightedness have a constant tension of accommodation ( that is, an increase in the refractive power of the lens), which makes it possible to compensate to a certain extent for the existing visual impairment. At the same time, when working with small details, the accommodation of a person with hyperopia is strained to the limit, as a result of which the muscles and tissues involved in this process quickly get tired, which leads to the appearance of characteristic symptoms.
Visual discomfort in patients with hyperopia can manifest itself:
- rapid fatigue;
- burning sensation in the eyes;
- gore in the eyes;
- increased tearing;
- photophobia ( all of the above symptoms are aggravated by bright light);
Grade clinical manifestations plays an important, but far from decisive, role in the diagnosis. To confirm the presence of hyperopia and prescribe correct treatment it is necessary to conduct a number of additional instrumental studies.
With hyperopia, the doctor may prescribe:
- measurement of visual acuity;
- determination of the degree of hyperopia;
- study of the refractive systems of the eye.
Measurement of visual acuity in hyperopia
Visual acuity is the ability of the human eye to distinguish between two separate points located at a certain distance from each other. In medical practice, it is considered normal if, from a distance of 5 meters, the human eye can distinguish 2 points that are 1.45 mm apart from each other.To assess the patient's visual acuity, special tables are used, on which letters or symbols of various sizes are displayed. The essence of the study is as follows. The patient enters the doctor's office and sits on a chair located 5 meters from the tables. After that, the doctor gives him a special opaque plate and asks him to cover one eye with it, and look at the table with the other eye ( the eye covered by the plate should remain open at the same time). After that, the doctor, using a thin pointer, begins to point to letters or symbols of certain sizes ( first to large, then to smaller), and the patient should name them.
If the patient can easily name the letters located in the 10th row of the table, then he has one hundred percent vision. Such results can be observed in healthy young people, as well as in patients with mild hyperopia, which is compensated by accommodation. With severe hyperopia, images of small objects become blurry, as a result of which the patient can only recognize larger letters.
After determining the visual acuity of one eye, the doctor asks to cover the other eye with a plate and repeats the procedure.
Determination of the degree of hyperopia
Determination of the degree of hyperopia can be carried out directly during the study of visual acuity. The essence of the method is as follows. After identifying the letters that the patient can no longer name correctly ( because it sees them odd), they put on his eyes special glasses, in which you can change glasses ( i.e. lenses). After that, the doctor inserts lenses with a certain refractive power into the glasses and asks the patient to evaluate the nature of the changes ( that is, is it better to see the letters on the table). Lenses with weaker refractive power are used first, and if this is not enough, stronger lenses are used ( each subsequent lens used in the diagnostic process must have a refractive power of 0.25 diopters more than the previous one).The doctor's opinion is based on the refractive power of the lens, which is necessary for the patient to be able to easily read the letters from the tenth row of the table. If, for example, this required a 1 diopter lens, then the patient has 1 diopter farsightedness.
Depending on the violation of the refractive system, the eyes are distinguished:
- Hyperopia weak degree - up to 2 diopters.
- Hyperopia medium - from 2 to 4 diopters.
- Hyperopia high degree - more than 4 diopters.
Types of farsightedness
The type of farsightedness is medical indicator, allowing to determine the severity of hyperopia and the compensatory possibilities of accommodation in a particular patient.With the development of hyperopia, images of visible objects are not focused directly on the retina, but behind it, and therefore are perceived by a person as blurry, indistinct. To compensate for this deviation, accommodation is included, which consists in changing ( enhancing) the refractive power of the lens. With mild hyperopia, this may be enough to compensate for the existing deviations, as a result of which a person will see objects quite clearly.
The more pronounced hyperopia, the more tension of accommodation is required to focus images on the retina. With the depletion of this compensatory mechanism (what is observed with high degree of hyperopia) a person will see poorly not only close, but also far-away objects. That is why the determination of the compensatory possibilities of accommodation of a patient with hyperopia is of particular importance.
With farsightedness, determine:
- Explicit hyperopia. This is the severity of hyperopia, determined when ( preserved) accommodation when the lens of the eye is functioning normally. Determination of obvious hyperopia is carried out during the study of visual acuity in the process of selecting corrective lenses.
- Complete hyperopia. This term denotes the severity of hyperopia, determined when the accommodation device is turned off. During the study, special drops are used ( atropine). Atropine causes persistent relaxation of the ciliary muscle, as a result of which the ligaments of the lens are tense and it is fixed in the maximally flattened state, when its refractive power is minimal.
- Latent hyperopia. Represents the difference between full and overt hyperopia, expressed in diopters. Latent hyperopia reflects the extent to which the compensatory capabilities of the lens are involved in a particular patient.
Study of the refractive systems of the eye
The research methods described above are subjective, that is, they are evaluated based on the patient's responses. However, to date, many techniques have been developed to investigate various functions eyes objectively, that is, more accurately.In the diagnosis of hyperopia, the following can be used:
- Skiascopy ( shadow test). The essence this study is as follows. The doctor sits in front of the patient and, at a distance of 1 meter from the examined eye, sets a special mirror that directs the beam of light directly to the center of the patient's pupil. Light is reflected from the retina of the examined eye and is perceived by the doctor's eye. If during the examination the doctor begins to rotate the mirror around a vertical or horizontal axis, a shadow may appear on the retina, the nature of the movement of which will depend on the state of the refractive system of the eye. With hyperopia, this shadow will appear from the side to which the mirror will be displaced. When this shadow is detected, the doctor places lenses with a certain refractive power in front of the mirror until this shadow disappears. Depending on the refractive power of the lens used, the degree of hyperopia is determined.
- Refractometry. To conduct this study, it is used special device- a refractometer, consisting of a light source, an optical system and a measuring scale. During the examination, the doctor directs a beam of light into the patient's pupil, while horizontal and vertical stripes appear on the retina. Normally, they intersect with each other, and with hyperopia, they diverge. In the latter case, the doctor begins to rotate a special handle, as a result of which he changes the refractive power of the device, which leads to a displacement of the lines on the patient's retina. At the moment when these lines intersect, the refractive power of the lens required to achieve this result is estimated, which determines the degree of farsightedness.
- Computerized keratotopography. This method is designed to study the shape, curvature and refractive power of the cornea. Research is carried out with the help of modern computer technology without causing any discomfort to the patient and not taking a lot of time ( on average, the procedure lasts from 3 to 5 minutes).
Correction and treatment of hyperopia
As mentioned earlier, with hyperopia, images of visible objects are not focused directly on the retina, but behind it. Consequently, to move the main focus to the retina in hyperopia, it is necessary to enhance the refractive power of the eye with the help of a collecting lens or replace the "defective" part of the refractive system ( if possible).Can farsightedness be cured?
To date, hyperopia is quite easily corrected using various techniques or even completely eliminated. At the same time, it is worth noting that with prolonged progression of the disease, as well as in the case of an incorrectly selected correction method, complications may develop, some of which may cause total loss vision.With hyperopia, you can use:
- glasses;
- laser treatment;
- lens replacement;
- surgical treatment.
Glasses for correcting hyperopia
Wearing glasses is one of the most common and available ways correction of hyperopia. The essence of the method is that a collecting lens with a certain refractive power is installed in front of the eye. This enhances the refraction of the rays passing through the lens and the refracting structures of the eye, as a result of which they ( rays) focus directly on the retina for clear images.The rules for prescribing glasses for hyperopia include:
- Selection of lenses for each eye separately. This procedure is usually performed in an ophthalmologist's office ( a doctor who diagnoses and treats eye diseases) during the determination of visual acuity and the degree of hyperopia.
- Use of a lens with maximum refractive power and high visual acuity. As mentioned earlier, when determining the degree of hyperopia, the doctor places lenses with different refractive powers in front of the patient's eye until the patient can easily read the letters from the tenth row of a special table. However, it should be remembered that in this case, explicit hyperopia is determined, that is, the accommodation apparatus is maximally tense. If for spectacle correction to use the very first lens that provided normal visual acuity, the person will see relatively well, but the refractive power of the lens will be maximum ( that is, accommodation will remain tense). That is why, when choosing glasses, the refractive power of the lenses must be increased until a person begins to see the tenth row of the table vaguely ( in this case, the refractive power of the lens will be minimal). After that, the lens is replaced with the previous one, which will be used to make glasses.
- Checking the pungency binocular vision. Even in the case of the correct selection of corrective lenses for each eye separately, it may turn out that after the production of glasses, objects visible through them will double. This deviation is usually caused by impairment of binocular vision ( that is, the ability to see a clear image with both eyes at the same time), which may be associated with various diseases. That is why, after fitting the lenses, you need to check right in the ophthalmologist's office whether the patient sees with both eyes ( there are many different tests for this).
- Lens tolerance test. After fitting corrective lenses, a person may experience certain discomfort In eyes ( tearing, pain, burning), Related abrupt change the state of the accommodation systems. This is why the patient must remain in the trial frame for a few minutes after fitting the lenses. If after that no deviations are observed, you can safely write out a prescription for glasses.
It is also worth noting that with hyperopia, you should start wearing glasses as early as possible, as this will eliminate discomfort ( associated with blurring of visible objects) and prevent the development of complications.
Does a child need glasses for hyperopia?
The need to wear glasses in children is due to the cause and degree of farsightedness. So, for example, if hyperopia is of a physiological nature, no correction is required, since the child's vision is independently normalized by the age of 13-14. At the same time, with severe hyperopia associated with deformation of the shape and size of the eyeball, as well as damage to the lens or cornea, the degree of hyperopia should be determined as soon as possible and glasses should be prescribed, since various complications in children develop much faster than in adults. ...The selection of glasses for children is carried out according to the same rules as for adults. However, it should be noted that as the child grows, the severity of hyperopia may decrease ( due to the growth of the eyeball, increasing the refractive power of the cornea and lens). That is why children under 14 are advised regularly ( semiannually) assess visual acuity, determine the degree of hyperopia and, if necessary, change lenses with glasses.
Contact lenses for hyperopia
Selection and appointment principle contact lenses the same as when assigning glasses. The main difference is the way they are used. Contact lenses are attached directly to the patient's eye ( on the anterior surface of the cornea), which ensures the correction of the refractive system of the eye. Using contact lenses is a more convenient and accurate method of vision correction than wearing glasses.The advantages of contact lenses over glasses are:
- Optimal vision correction. When using glasses, the distance between the refractive lens and the retina of the eye is constantly changing ( when turning the eyes to the side, when moving away or approaching glasses). The contact lens is fixed directly on the cornea, as a result of which the distance from it to the retina remains constant. Also, the lens moves at the same time as the eyeball, which contributes to obtaining an even clearer image.
- Practicality. Contact lenses do not fog up when moving from a cold room to a warm one, do not get wet in the rain, and do not fall out when tilting the head, running or other active movements. This is why wearing contact lenses allows a person to lead a more active lifestyle than wearing glasses.
- Aesthetics. High-quality contact lenses are almost invisible and do not cause any cosmetic inconvenience to a person, which cannot be said about glasses.
Laser correction of hyperopia
Treatment of hyperopia with the help of modern laser technologies allows in some cases to eliminate the existing visual defect, and this can be done quite quickly, safely and painlessly.Laser correction of hyperopia includes:
- Photorefractive keratectomy ( PRK). The essence this method lies in the fact that with the help of a special laser, removal ( evaporation) of the upper layer of the cornea ( refractive stroma), as a result of which ( intensifies) its refractive power. This allows you to reduce the degree of hyperopia and reduce the load on the accommodative system of the eye. The advantages of this method include safety and high efficiency ( with mild to moderate hyperopia). The disadvantage of this method is the long ( up to 1 month) the recovery period and the possibility of corneal opacity in postoperative period, which is associated with damage to its upper ( epithelial) layer.
- Transepithelial photorefractive keratectomy ( trans-PRK). The difference between this method and conventional PRK is less trauma to the upper ( epithelial) layer of the cornea. This makes the procedure more convenient ( the patient experiences less discomfort than with conventional PRK), shorten the recovery period to 2 - 3 weeks and reduce the risk of complications ( including corneal opacities) in the postoperative period.
- Laser keratomileusis. This is a modern high-tech method that allows you to eliminate farsightedness up to 4 diopters. The essence of the method is as follows. Using a laser, an incision is made on the anterior surface of the cornea, after which a flap is formed, consisting of superficially located epithelium and other tissues. This flap is lifted, exposing the stroma itself. After that, it is produced laser removal stroma, necessary to normalize the refractive system of the eye. Then the detached flap returns to its place, where it is fixed almost instantly due to its plastic properties. As a result of such manipulation, the epithelial layer of the cornea is practically not damaged, which prevents the development of complications inherent in PRK and trans-PRK. The laser keratomileusis procedure itself lasts a few minutes, after which the patient can go home. After that, no sutures, scars and opacities remain on the cornea.
Lens replacement for hyperopia
Using this method, even severe hyperopia associated with damage to the lens can be eliminated ( including with presbyopia). The essence of the method is that the old lens is removed from the eye, and a new lens is placed in its place ( artificial, which is a lens with a certain refractive power).The operation itself lasts no more than half an hour and is performed under local anesthesia, but in some cases ( at emotional instability patient, when replacing the lens of a child) it is possible to use special drugs that introduce the patient into a medical sleep. In the latter case, the length of stay of the patient in the hospital after surgery can increase from several hours to several days.
The first stage of the operation is to remove the old lens. To do this, the doctor makes a small ( length about 2 mm) incision, after which, with the help of a special ultrasound machine, it turns the lens into an emulsion ( liquid) and removes it. Then an artificial lens is inserted into the place of the lens, which itself straightens and is fixed in the desired position. Then the incision in the corneal area is sutured with the finest threads, and after several hours of observation, the patient can go home. After the procedure, it is recommended to visit an ophthalmologist several times a month to assess visual acuity and timely detection possible complications (suture divergence, lens displacement, infection, and so on).
Farsightedness surgery
Surgical treatment of hyperopia is indicated when it is impossible to correct or eliminate given state other, less traumatic methods.Surgical treatment for hyperopia includes:
- Phakic lens implantation. The essence of the method is that a specially selected ( according to all the rules for the selection of lenses for hyperopia) the lens is implanted under the cornea and attached to its posterior wall. As a result, the same clinical effect is achieved as when using conventional contact lenses ( that is, the refractive power of the cornea increases and visual acuity is normalized). This eliminates a number of unpleasant moments associated with the use of the latter ( in particular, there is no need for regular lens replacement since phakic lenses can last for many years). The disadvantages of the method include the fact that in the case of progression of the disease and an increase in the degree of hyperopia ( what can be observed with presbyopia) you will have to remove the old lens and install a new one or use other methods of vision correction ( particularly contact lenses or glasses).
- Radial keratotomy. The essence of this method is as follows. Using a special scalpel, several radial ( heading from the pupil to the periphery) cuts. After fusion, these incisions change the shape of the cornea, that is, increase its curvature, which leads to an increase in refractive power. It should be noted that due to the long recovery period, the risk of damage to the cornea during surgery and frequent postoperative complications, this technique is practically not used today.
- Keratoplasty. The essence of this method is the transplantation of the donor cornea, which was previously processed using special techniques ( that is, she was given special form providing the necessary refractive power). Donor cornea can be implanted ( take root) directly into the patient's cornea, attach on its outer surface or completely replace it.
Hyperopia prevention
Prevention is a set of measures aimed at preventing the development of a disease or slowing down the rate of its progression. Since farsightedness in most cases is caused by anatomical changes in the eyeball, cornea or lens, it is almost impossible to prevent its development. At the same time, adherence to certain rules and recommendations will slow down the progression of the disease and reduce the likelihood of complications. Farsightedness prevention includes:
- Timely and correct correction of hyperopia. This, perhaps, is the first and main measure to alleviate the course of the disease. Immediately after the diagnosis is made, you should discuss with the doctor possible ways to eliminate the existing defect, and if this is not possible, choose optimal method correction ( with glasses, contact lenses and so on).
- Elimination of excessive visual stress. With hyperopia ( without correction) there is a constant tension of the ciliary muscle, which leads to an increase in the refractive power of the lens and makes it possible to compensate to a certain extent for the existing defect. However, prolonged reading or working at a computer leads to overfatigue of accommodation, as a result of which a person experiences visual discomfort, burning or pain in the eyes, increased tearing and so on. To prevent this, it is recommended to regularly ( every 15 - 20 minutes) take a short break, during which you should move away from the workplace, walk around the house or perform several simple exercises for eyes.
- Proper lighting of the workplace. As mentioned earlier, working in low light can contribute to the development of visual discomfort, burning and pain in the eyes. This is why all people, and especially hyperopic patients, should be properly illuminated. workplace... It is best to work in natural daylight with the table close to the window. If you need to work at night, remember that direct light ( directed from the lamp directly to the workplace) extremely adversely affects the eyes. It is best to use bounced light, for which you can direct the lamp at a white ceiling or wall. Also, when working at a computer, it is recommended to turn on a lamp or an ordinary lamp ( that is, do not work in complete darkness), as the pronounced contrast between a bright monitor and a dark room significantly increases eye strain.
- Regular tests of visual acuity. Even after the selection of corrective glasses or the elimination of hyperopia with the help of other methods, it is recommended to regularly ( 1 - 2 times a year) visit an optometrist. This will allow timely identification of various deviations ( for example, progression of presbyopia) and timely prescribe treatment.
Exercises ( gymnastics) for eyes with farsightedness
There are many exercises that help reduce eye strain and normalize blood microcirculation in the ciliary muscle, thereby slowing the progression of hyperopia, reducing the severity of clinical manifestations and preventing the development of complications.A set of exercises for hyperopia includes:
- Exercise 1. You should find the most distant point on the horizon ( the roof of the house, wood and so on) and look at it for 30 - 60 seconds. This will reduce the load on the ciliary muscle and improve blood microcirculation in it, thereby reducing the likelihood of developing visual discomfort.
- Exercise 2. The exercise is performed while standing by a window or outside. First, you should try to focus your vision on a closely located object ( for example, on the tip of the nose), and then look into the distance ( as far as possible), and then repeat the procedure.
- Exercise 3. In case of fatigue during reading, it is recommended to put the book aside and close your eyes tightly several times in a row, holding them in this position for 2 - 4 seconds. This exercise improves microcirculation in the muscles of the eye, and also helps to temporarily relax accommodation.
- Exercise 4. You need to close your eyes and slowly rotate your eyeballs clockwise and then in the opposite direction.
Complications of hyperopia
As mentioned earlier, long-term progression of hyperopia without appropriate correction can lead to a number of formidable complications. Nonspecific complications of hyperopia include an infectious lesion of the cornea ( keratitis), conjunctiva ( conjunctivitis), century ( blepharitis). This can be facilitated by a violation of microcirculation in the structures of the eye, associated with constant voltage accommodation and visual fatigue. Also, hyperopia can be complicated:
- spasm of accommodation;
Strabismus with farsightedness
Strabismus is called pathological condition, in which the pupils of both eyes "look" in different directions... With farsightedness, convergent strabismus may develop, in which the pupils of the eyes are excessively deflected towards the center. The reason for the development this complication lies in the physiology of the visual analyzer. Under normal conditions, with the voltage of the accommodation apparatus ( that is, with an increase in the refractive power of the lenses), natural convergence is noted, that is, the convergence of the pupils of both eyes. In a healthy person, this mechanism allows you to more accurately focus the gaze on a closely located object.With severe hyperopia, there is a constant compensatory voltage of accommodation ( that is, contraction of the ciliary muscle and an increase in the refractive power of the lens), as a result of which convergence also occurs. Initially, this condition is easily eliminated by using lenses correcting hyperopia. With a long-lasting tension of accommodation and the accompanying convergence, an irreversible change in the oculomotor muscles may occur, due to which the strabismus will become permanent ( what is most important in children).
Amblyopia ( lazy eye) with hyperopia
The essence this disease consists in a decrease in visual acuity even with optimal correction of hyperopia using lenses, and it is not possible to identify any other anatomical defects in the organ of vision. In other words, "lazy eye" is functional impairment, which occurs with prolonged progression of high-grade hyperopia.With timely detection and initiation of appropriate treatment, amblyopia can be eliminated ( treatment should be combined with adequate correction of hyperopia), however, the longer this state persists, the harder it will be to restore normal function eyes further.
Spasm of accommodation with hyperopia
The essence of this complication lies in a long and pronounced reduction ( spasm) the ciliary muscle, which temporarily loses its ability to relax. This is manifested by the inability to focus vision on objects located at different distances from the eye.In a healthy person, accommodation spasm can develop during prolonged work at the computer or when reading, that is, in the case when there is a prolonged tension of accommodation and overwork of the ciliary muscle. However, with severe hyperopia, accommodation is tense almost constantly, as a result of which the risk of spasm development increases significantly. That is why it is extremely important to start correcting and treating hyperopia in a timely manner.
With the development of a spasm of accommodation, it is recommended to interrupt the work being performed and do several exercises to relax the eyes. In case of severe spasm, you should consult a doctor ( ophthalmologist). If necessary, the doctor can drop special drops into the patient's eyes ( e.g. atropine), as a result of which the opposite phenomenon will occur - the ciliary muscle will relax and fix in this position for several hours or days, that is, accommodation paralysis will occur.
Myopia with hyperopia
Myopia is a pathological condition in which a person is ill ( not clear) sees distant objects. Usually myopia develops as an independent disease ( what poor eye hygiene can contribute to), and can also occur with long-term and uncorrected hyperopia.The mechanism of myopia development is as follows. When focusing on a closely located object, the fibers of the ciliary muscle contract, the ligaments of the lens relax and increase it ( lens) refractive power. When vision moves to a more distant object, the ciliary muscle relaxes, the lens flattens, and its refractive power decreases. However, with a long, continuous voltage of accommodation ( which is observed with uncorrected hyperopia) there is a gradual hypertrophy ( that is, an increase in size and strength) of the ciliary muscle. In this case, when accommodation is relaxed, the muscle itself relaxes only partially, as a result of which the ligaments of the lens remain in a relaxed position, and the refractive power of the lens remains increased.
It should be noted that the development of myopia in hyperopia is a long-term process that progresses over several years. At the same time, if myopia has developed, a person will have difficulty seeing both close and distant objects, that is, his visual acuity will progressively deteriorate. In this case, only one vision correction ( with glasses or lenses
Myopia makes life difficult for many modern people.
This pathology contributes to the elongation of the eyeball, so the rays of light do not reach the retina and are focused in front of it. This becomes the reason that a person does not see well into the distance.
The attributes of the contact correction method have different basic radii of curvature, diameter and number of diopters.
Features of the selection of contact lenses for the correction of myopia
In order to prevent myopia from developing and progressing, it is necessary to start using the correction in time.
Correct selection of correction is the key to good vision
Important: At the initial stage, when the disease has not yet reached the -1 D limit, it is not recommended to use the contact method.
Constant correction can provoke deterioration.
If the patient has myopia with a deviation from the norm from -1 D and more, then the main way to stop the progression of pathology is precisely contact correction.
It is also worth noting that this type of correction is not suitable for children. This is due to the fact that myopic babies will not be able to use contact products on their own.
The glasses are over suitable method improvement of vision in childhood myopia. Explore the selection rules in this article
There are such rules for the selection of contact correction for myopia:
- It is best to choose products from silicone hydrogel.
- Central thickness therapeutic corrective attribute depends on the number of required diopters.
- Product diameter should be suitable for the individual parameters of the patient's eye. In order to determine this parameter, the ophthalmologist applies computer diagnostics organs of vision.
- Healing attribute should be scattering and have minus specifications.
- Selection of the right cylinder axes if myopia is complicated by astigmatism.
- Choice wearing mode... These can be lenses that need to be removed at night and worn throughout the day. There is also an option of night lenses or permanent lenses that can be worn 30 and more days without taking off.
- By the nature of the design and shape, you need to choose spherical... If astigmatism is present, then it will do toric option. When presbyopia is noted, the doctor prescribes multifocal products.
Only an ophthalmologist can say for sure which lenses are best for myopia.
Before selection, an optometrist in mandatory conducts diagnostics and only on the basis of the results of the examination, he makes a final conclusion about the nature of the correction.
Features and benefits of using lenses for myopia
Medicine is actively developing. Today it is possible to get rid of myopic disorder permanently with laser surgery.
However, even despite this, lenses for correcting myopia remain relevant due to such positive properties:
- they do not limit the visual field;
- they can be worn with sunglasses at the same time;
- ideal for active pastime;
- there is no glare;
- they don't fog up;
- the image is not distorted;
- they do not slip, unlike glasses;
- have a protective property against ultraviolet radiation.
Those who choose this method of improving vision should also familiarize themselves with its features:
- putting on the product requires training and special skills;
- addiction occurs gradually;
- the healing attribute can slip out of hands and get lost;
- it is necessary to learn how to properly care for and disinfect the product.
Fact: If hygiene and disinfection rules are not followed, complications in the form of inflammatory processes may occur.
If you use contact correction right, then it will greatly facilitate life and relieve the inconvenience associated with poor eyesight.
Check out also a video clip on this topic:
Biconvex lens
Plano-convex lens
Characteristics of thin lenses
Depending on the forms, there are collective(positive) and scattering(negative) lenses. The group of collecting lenses usually includes lenses in which the middle is thicker than their edges, and the group of scattering lenses is lenses whose edges are thicker than the middle. It should be noted that this is only true if the refractive index of the lens material is greater than that of environment... If the refractive index of the lens is lower, the situation will be reversed. For example, an air bubble in water is a biconvex diffusing lens.
Lenses are characterized, as a rule, by their optical power (measured in diopters), or focal length.
For building optical instruments with corrected optical aberration (primarily chromatic due to light dispersion - achromats and apochromats), other properties of lenses / lens materials are also important, for example, refractive index, dispersion coefficient, material transmittance in the selected optical range.
Sometimes lenses / lens optical systems (refractors) are specifically designed for use in environments with a relatively high refractive index (see immersion microscope, immersion liquids).
Lens types:
Gathering:
1 - biconvex
2 - plano-convex
3 - concave-convex (positive meniscus)
Scattering:
4 - biconcave
5 - flat-concave
6 - convex-concave (negative meniscus)
The convex-concave lens is called meniscus and can be collective (thickens towards the middle) or scattering (thickens towards the edges). A meniscus whose surface radii are equal has optical power equal to zero (used for dispersion correction or as a cover lens). So, the lenses of glasses for myopia are usually negative menisci.
A distinctive feature of a collecting lens is the ability to collect rays incident on its surface at one point located on the other side of the lens.
The main elements of the lens: NN - the main optical axis - a straight line passing through the centers spherical surfaces limiting the lens; O - optical center - a point that for biconvex or biconcave (with the same surface radii) lenses is located on the optical axis inside the lens (in its center).
Note... The ray path is shown as in an idealized (flat) lens, without indicating refraction at the real interface. Additionally, a somewhat exaggerated image of a biconvex lens is shown.
If a luminous point S is placed at some distance in front of the collecting lens, then the light beam directed along the axis will pass through the lens without refracting, and the rays passing not through the center will be refracted towards the optical axis and intersect on it at some point F, which and will be the image of point S. This point is called conjugate focus, or simply focus.
If light falls on the lens from a very distant source, the rays of which can be represented as a parallel beam, then upon exiting it, the rays will be refracted at a larger angle and the point F will move on the optical axis closer to the lens. Under these conditions, the point of intersection of the rays emerging from the lens is called main focus F ', and the distance from the center of the lens to the main focus is the main focal length.
Rays falling on a diffusing lens, upon leaving it, will be refracted towards the edges of the lens, that is, scattered. If these rays continue in the opposite direction as shown in the figure by the dotted line, then they will converge at one point F, which will be focus this lens. This focus will imaginary.
Imaginary focus of a diffusing lens
What has been said about the focus on the main optical axis equally applies to those cases when the image of a point is located on the side or inclined optical axis, i.e., a line passing through the center of the lens at an angle to the main optical axis. The plane perpendicular to the main optical axis located at the main focus of the lens is called main focal plane, and in conjugate focus - just focal plane.
The collecting lenses can be directed to the object by either side, as a result of which the rays, after passing through the lens, can be collected from one side or the other. Thus, the lens has two focuses - front and rear... They are located on the optical axis on both sides of the lens at the focal length from the center of the lens.
Imaging with a thin converging lens
When describing the characteristics of the lenses, the principle of constructing an image of a luminous point at the focus of the lens was considered. The rays falling on the lens from the left pass through its back focus, and those falling on the right through the front focus. It should be noted that in diffusing lenses, on the contrary, the back focus is located in front of the lens, and the front focus is behind.
The construction of an image of objects with a certain shape and size by the lens is obtained as follows: for example, the AB line is an object located at a certain distance from the lens, significantly exceeding its focal length. An innumerable number of rays will pass from each point of the object through the lens, of which, for clarity, the figure schematically shows the path of only three rays.
Three rays emanating from point A will pass through the lens and intersect at the corresponding vanishing points on A 1 B 1, forming an image. The resulting image is valid and inverted.
In this case, the image was obtained in a conjugate focus in a certain focal plane FF, somewhat distant from the main focal plane F'F ', passing parallel to it through the main focus.
If the object is at a distance infinitely far from the lens, then its image is obtained in the back focus of the lens F ' valid, inverted and reduced to the semblance of a point.
If the object is close to the lens and is at a distance exceeding twice the focal length of the lens, then its image will be valid, inverted and reduced and will be located behind the main focus in the segment between it and the double focal length.
If the subject is placed at double focal length from the lens, then the resulting image is on the other side of the lens at double focal length from it. The image is obtained valid, inverted and equal in size subject.
If the subject is placed between the front focus and double focal length, then the image will be obtained at double focal length and will be valid, inverted and increased.
If the object is in the plane of the front main focus of the lens, then the rays, passing through the lens, will go parallel, and the image can be obtained only at infinity.
If the object is placed at a distance less than the main focal length, then the rays will come out of the lens in a diverging beam, without crossing anywhere. The image is thus obtained imaginary, direct and increased That is, in this case, the lens works like a magnifying glass.
It is easy to see that when an object approaches from infinity to the front focus of the lens, the image moves away from the back focus, and when the object reaches the front focus plane, it turns out to be at infinity from it.
This pattern has great importance in practice different types photographic work, therefore, to determine the relationship between the distance from the object to the lens and from the lens to the image plane, it is necessary to know the basic lens formula.
Thin lens formula
Distances from the point of the subject to the center of the lens and from the point of the image to the center of the lens are called conjugate focal lengths.
These quantities are dependent on each other and are determined by a formula called formula thin lens :
where is the distance from the lens to the object; - distance from the lens to the image; is the main focal length of the lens. In the case of a thick lens, the formula remains unchanged with the only difference that the distances are measured not from the center of the lens, but from the main planes.
To find one or another unknown quantity for two known ones, the following equations are used:
It should be noted that the signs of the quantities u , v , f are selected on the basis of the following considerations - for a real image from a real object in a collecting lens - all these values are positive. If the image is imaginary, the distance to it is taken to be negative, if the object is imaginary, the distance to it is negative, if the lens is scattering, the focal length is negative.
Image scale
The scale of the image () is the ratio of the linear dimensions of the image to the corresponding linear dimensions of the object. This ratio can be indirectly expressed as a fraction, where is the distance from the lens to the image; - the distance from the lens to the object.
There is a reduction factor, that is, a number that shows how many times the linear dimensions of the image are less than the actual linear dimensions of the object.
In the practice of calculations, it is much more convenient to express this ratio in terms of or, where is the focal length of the lens.
.
Calculation of the focal length and optical power of the lens
The lenses are symmetrical, that is, they have the same focal length regardless of the direction of the light - left or right, which, however, does not apply to other characteristics, for example, aberrations, the magnitude of which depends on which side of the lens is turned towards the light.
Multi-lens combination (centered system)
Lenses can be combined with each other to build complex optical systems. The optical power of a system of two lenses can be found as a simple sum of the optical powers of each lens (provided that both lenses can be considered thin and they are located close to each other on the same axis):
.
If the lenses are located at some distance from each other and their axes coincide (a system of an arbitrary number of lenses with this property is called a centered system), then their total optical power can be found with a sufficient degree of accuracy from the following expression:
,
where is the distance between the principal planes of the lenses.
Disadvantages of a simple lens
In modern photographic equipment, high demands are made on image quality.
The image provided by a simple lens, due to a number of shortcomings, does not meet these requirements. Elimination of most of the shortcomings is achieved by the appropriate selection of a number of lenses in a centered optical system - a lens. Images obtained with simple lenses have various disadvantages. The disadvantages of optical systems are called aberrations, which are divided into the following types:
- Geometric aberration
- Diffractive aberration (this aberration is caused by other elements of the optical system, and has nothing to do with the lens itself).
Lenses with special properties
Organic polymer lenses
Contact lenses
Quartz lenses
Quartz glass is pure remelted silica with insignificant (about 0.01%) additions of Al 2 O 3, CaO and MgO. It is characterized by high thermal stability and inertness to many chemicals, with the exception of hydrofluoric acid.
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