Types of lenses for glasses and how to make the right choice. Biconvex lens Laser correction of farsightedness

Lesson objectives: formation of ideas about the structure of the eye and the mechanisms of operation of the optical system of the eye; clarification of the conditionality of the structure of the optical system of the eye by the laws of physics; developing the ability to analyze the phenomena being studied; developing a caring attitude towards your health and the health of others.

Equipment: table “Organ of vision”, model “Human Eye”; light-collecting lens, high-curvature lens, low-curvature lens, light source, task cards; on the students' tables: a light-gathering lens, a light-diffusing lens, a screen with a slot, a light source, a 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 vision?
– Which organ 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 desk - a model of the "Human Eye". Having collected cards with student answers, the biology teacher checks their completion, together with the students naming and showing the parts of the eye on the model and poster.

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, turning 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 reminds you of a lens?

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 a lens are concentrated at one point called the focus. A biconcave lens is a diverging lens; rays passing through the lens are scattered in such a way that the continuation of the rays is collected at an imaginary focus.

(Physics teacher draws(rice. 1) on the board, and students in their notebooks, the path of rays in a converging and diverging lens.)

Rice. 1. Path of rays in collecting and diverging lenses (F – focus)

Physics teacher. What will the image be like if the object is located behind the double focal length of the converging lens?

(Students draw in their notebooks the course of the rays in this case (Fig. 2) and make sure that the image is reduced, real, inverted.)

Rice. 2. Constructing an image in a collecting lens

Frontal experiment

On each table, students have a converging and diverging lens, a current source, an electric light bulb on a stand, a screen with a slot in the shape of the letter L, and a screen.

The physics teacher asks students to choose a biconvex, i.e. converging lens and verify experimentally that the converging 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 the inverted letter G.

(Students are convinced by experience that the image is actually inverted and appears clearly on the screen only at a certain location of the screen 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 body are a converging 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 not upside down?

Students. Normal, and not inverted, vision of objects is due to their repeated “inversion” in the cortical region visual analyzer.

Biology teacher. We see objects well at different distances. This occurs thanks 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 shorter the focal length - such lenses are called short-focus lenses, lenses with small curvature, i.e. with a large radius of curvature are called long-focus (Fig. 4).

Rice. 4. Change in lens properties depending on its curvature

Biology teacher. When viewing nearby objects, the radius of curvature of the lens decreases, and it acts as a short-focus lens. When viewing distant objects, the radius of curvature of the lens increases and it acts as a long-focus lens. In both cases, this is necessary to ensure that the image is always focused on the retina. The ability to clearly see objects at different distances due to changes in the curvature of the lens is called accommodation (students write down the definition in their notebooks).

There are deviations in the structure of the eye or in the functioning of the lens.

In myopia, the image is focused in front of the retina due to excessive curvature of the lens or elongation of the eye axis. With farsightedness, the image is focused behind the retina due to insufficient curvature of the lens or a shortened axis of the eye.

Physics teacher. Which lenses are needed to correct myopia and which ones to correct farsightedness?

Students. Myopia is a diverging lens, farsightedness is a converging lens.

(A physics teacher experimentally proves the validity of students’ conclusions by demonstrating experience..)

Biology teacher. There is another deviation from the norm in the operation of the optical system of the human eye - astigmatism. Astigmatism is the impossibility of all rays converging at one point, at one focus. This occurs due to deviations of the corneal curvature from spherical. Cylindrical lenses are used to correct astigmatism.

conclusions

Students, together with the biology teacher, formulate the basic rules of visual hygiene:

– protect eyes from mechanical influences;
– 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 can’t lean close to the book, because this can lead to the development of myopia;
– cannot be 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.

Thank you

The site provides background information for informational purposes only. Diagnosis and treatment of diseases must be carried out under the supervision of a specialist. All drugs have contraindications. Consultation with a specialist is required!

What is farsightedness?

Farsightedness is a disease of the eye characterized by damage to its refractive system, as a result of which images of nearby objects are focused not on the retina ( as normal), and behind her. With farsightedness, people see the outlines of objects as indistinct, blurry, and the closer an 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 farsightedness, certain knowledge about the structure and functioning of the eye is necessary.

Conventionally, the human eye is divided into two sections - the retina and the refractive system of the eye. The retina is peripheral section visual analyzer, consisting of many light-sensitive nerve cells. Photons ( light particles), reflected from various surrounding objects, fall on the retina. As a result of this, 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, having the shape of a hemisphere. The cornea has a constant refractive power of approximately 40 diopters ( diopter - a unit of measurement that determines the degree of refractive power of a lens).
Lens. It is located behind the cornea and is a biconvex lens, which 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. This is a liquid located in special chambers of the eye in front 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 body. A clear, jelly-like substance that fills the space between the lens and the retina. Refractive power vitreous also insignificant. Its main function is to maintain the correct shape of the eye.

IN normal conditions When passing through the refractive system of the eye, all rays of light are collected ( focus) directly onto the retina, as a result of which a person can see a clear image of the observed object. If this object is far away, the refractive power of the lens changes ( that is, it decreases), causing the subject in question to become clearer. When viewing a nearby object, the refractive power of the lens increases, which also makes it possible to obtain a clearer image on the retina. This mechanism, which ensures clear vision of objects at different distances from the eye, is called accommodation ( device) eyes.

The essence of farsightedness is that beams of light 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 is unclear and blurry.

Reasons for the development of farsightedness

The cause of farsightedness can be either damage to the refractive structures of the eye or an irregular shape of the eyeball itself.

Depending on the cause and mechanism of development, the following are distinguished:

physiological farsightedness in children;
congenital farsightedness;
acquired farsightedness;
age-related farsightedness ( presbyopia).

Physiological farsightedness in children

The structure of the eye in a newborn differs 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 ability of the lens. As a result of these features, the image in children's eyes is projected not directly onto the retina, but behind it, which leads to farsightedness.

Almost all newborn children have physiological farsightedness of approximately 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 and the curvature increases ( and refractive power) cornea and lens. All this leads to the fact that at the age of 7 - 8 years the degree of farsightedness is only 1.5 - 2 diopters, and by the age of 14 ( when the formation of the eyeball is completed) For most teenagers, vision becomes completely normal.

Congenital farsightedness

Diagnose congenital ( pathological) farsightedness is possible only in children over 5–6 years of age, since up to this age the eyeball itself and the refractive structures of the eye continue to develop. At the same time, if a child aged 2–3 years has farsightedness of 5–6 diopters or more, there is a high probability that this phenomenon will not go away on its own as you grow older.

The cause of congenital farsightedness can be various anomalies of the eyeball or the refractive system of the eye.

Congenital farsightedness may result from:

Developmental disorders of the eyeball. If the eyeball is underdeveloped ( too small) or if its form is initially broken, subsequently ( as the child grows) it can also develop incorrectly, as a result of which the child’s farsightedness does not disappear, but may even progress.
Disorders of corneal development. As mentioned earlier, as a child grows older, the refractive power of his cornea increases. If this does not happen, the child will remain farsighted. Also more pronounced farsightedness ( more than 5 diopters) may occur in children with congenital abnormalities of the cornea ( that is, if the cornea is initially too flat, and its refractive power is extremely low).
Disorders of lens development. IN this group include congenital displacement of the lens ( when it is not located in its usual place), microphakia ( lens too small) and aphakia ( congenital absence of the lens).

Acquired farsightedness

Acquired farsightedness 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 caused by eye injuries that were performed incorrectly. surgical operations, tumors in the orbital area ( during growth, they can compress the eyeball, changing its shape). Also, the cause of farsightedness 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 opacities) and so on.

Age-related farsightedness ( presbyopia)

A separate form of acquired hypermetropia is age-related ( senile) farsightedness. 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 suspended just 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 vision on distant objects. When viewing objects close up, the opposite process occurs - tension in the ciliary muscle leads to relaxation ligamentous apparatus the 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 caused by 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 toward 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 farsightedness). By about 40 years of age, the developing nucleus becomes so dense that it disrupts the elasticity of the lens itself. In this case, when the ligaments of the lens are strained, the lens itself flattens only partially, which is due to the dense core located in its center. By the age of 60, the core becomes sclerotic, that is, it reaches its maximum density.

It is worth noting that the process of development of age-related farsightedness begins as early as early childhood, however, it becomes clinically noticeable only by the age of 40, which is manifested by a weakening of accommodation. It has been estimated that as a result of the formation and hardening of the lens nucleus, its accommodative capacity decreases by approximately 0.001 diopters daily from birth until age 60.

Symptoms, signs and diagnosis of farsightedness

With congenital ( not physiological) farsightedness, a child may not show any complaints for a long time. This is due to the fact that from the moment of birth he sees nearby objects blurry and does not know that this is not normal. In this case, parents may suspect hypermetropia based on the child’s characteristic behavior ( The child has difficulty distinguishing between closely located 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 typical for age-related farsightedness. The main complaint of such patients is the inability to clearly see nearby objects. This condition is aggravated in 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 move the book to arm’s length ( the need to do this regularly irritates many patients, which they mention when talking to their doctor).

To 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, when reading, the refractive power of the lens increases slightly, which allows you to focus your gaze on nearby text. However, people with farsightedness experience a constant strain of accommodation ( that is, an increase in the refractive power of the lens), which allows, to a certain extent, to compensate for the existing visual impairment. At the same time, when working with small parts, the accommodation of a person with farsightedness 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 farsightedness can manifest itself:

fatigue;
burning in the eyes;
pain in the eyes;
increased tearfulness;
photophobia ( all of the above symptoms are worse in bright light);

These manifestations may appear several minutes or hours after starting work with closely located objects and disappear some time after stopping this work. The speed of occurrence, as well as the severity and duration of symptoms depends on the degree of farsightedness ( the higher it is, the faster accommodation “gets tired” and the more pronounced the clinical manifestations of the disease).

Grade clinical manifestations plays an important, but far from decisive role in making a diagnosis. To confirm the presence of farsightedness and prescribe correct treatment it is necessary to conduct a number of additional instrumental studies.

For farsightedness, the doctor may prescribe:

measurement of visual acuity;
determination of the degree of hypermetropia;
study of the refractive systems of the eye.

Measuring visual acuity for farsightedness

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 separated by 1.45 mm.

To assess the patient's visual acuity, special tables are used that display letters or symbols of various sizes. 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 this, 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 must remain open). After this, the doctor, using a thin pointer, begins to point to letters or symbols of certain sizes ( first to large ones, then to smaller ones), and the patient must 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 degree hypermetropia, which is compensated by accommodation. With severe farsightedness, 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.

Determining the degree of hypermetropia

The degree of hypermetropia can be determined directly during a visual acuity study. The essence of the method is as follows. After identifying the letters that the patient can no longer name correctly ( because he sees them oddly), they put it over his eyes special glasses, in which you can change glass ( that is, lenses). After this, 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, has he become better able to see the letters on the table). Initially, lenses with weaker refractive power are used, 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 greater than the previous one).

The doctor's conclusion is based on the refractive power of the lens 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 lens with a power of 1 diopter, then the patient has farsightedness of 1 diopter.

Depending on the violation of the refractive system of the eye, there are:

Hypermetropia weak degree – up to 2 diopters.
Hypermetropia medium degree – from 2 to 4 diopters.
Hypermetropia high degree – more than 4 diopters.

The degree of hypermetropia is also determined for each eye separately.

Types of farsightedness

The type of farsightedness is medical indicator, allowing to determine the severity of hypermetropia and compensatory possibilities of accommodation in a particular patient.

With the development of farsightedness, images of visible objects are focused not directly on the retina, but behind it, and therefore are perceived by a person as blurry and indistinct. To compensate for this deviation, accommodation is activated, which consists in changing ( strengthening) refractive power of the lens. With weak hypermetropia, this may be enough to compensate for existing deviations, as a result of which the person will see objects quite clearly.

The more pronounced the hypermetropia, the greater the accommodation voltage required to focus images on the retina. When this is depleted compensatory mechanism (what is observed with high degree hypermetropia) a person will see poorly not only close, but also distant objects. That is why determining the compensatory possibilities of accommodation of a patient with farsightedness is of particular importance.

For farsightedness, it is determined:

Obvious hypermetropia. This is the severity of hypermetropia, determined when ( saved) accommodation, when the lens of the eye functions normally. Obvious hypermetropia is determined during a visual acuity study during the selection of corrective lenses.
Complete hypermetropia. This term denotes the severity of hypermetropia, determined when the accommodation apparatus 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 become tense and it is fixed in the most flattened state, when its refractive power is minimal.
Hidden hypermetropia. It is the difference between complete and obvious hypermetropia, expressed in diopters. Hidden 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 assessed based on the patient’s responses. However, to date, many techniques have been developed to study various functions eyes objectively, that is, more accurately.

In the diagnosis of hypermetropia, the following can be used:

Skiascopy ( shadow test). The essence this study is as follows. The doctor sits opposite the patient and, at a distance of 1 meter from the eye being examined, installs a special mirror that directs a beam of light directly to the center of the patient’s pupil. The light is reflected from the retina of the eye being examined 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 hypermetropia, this shadow will appear on the side towards which the mirror will shift. When a given shadow is identified, the doctor places lenses with a certain refractive power in front of the mirror until the shadow disappears. Depending on the refractive power of the lens used, the degree of hypermetropia is determined.
Refractometry. To conduct this study, we use 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, causing horizontal and vertical stripes to appear on the retina. Normally they intersect with each other, but with farsightedness they diverge. In the latter case, the doctor begins to rotate a special handle, as a result of which the refractive power of the device changes, which leads to a shift in 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.
Computer keratotopography. This method is designed to study the shape, curvature and refractive power of the cornea. The research is carried out using modern computer technology without causing any discomfort to the patient and without taking much time ( On average, the procedure lasts from 3 to 5 minutes).

Correction and treatment of farsightedness

As mentioned earlier, with farsightedness, images of visible objects are focused not directly on the retina, but behind it. Therefore, to move the main focus to the retina with hypermetropia, it is necessary to strengthen the refractive ability of the eye using a collecting lens or replace the “defective” part of the refractive system ( if possible).

Is it possible to cure farsightedness?

Today, farsightedness can be 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 can cause total loss vision.

For farsightedness you can use:

glasses;
laser treatment;
lens replacement;
surgical treatment.

Glasses for farsightedness correction

Wearing glasses is one of the most common and available ways correction of farsightedness. 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 rays passing through the lens and the refractive structures of the eye, as a result of which they ( rays) focus directly on the retina for clear images.

Rules for prescribing glasses for farsightedness 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 determination of visual acuity and degree of hypermetropia.
Using a lens that has maximum refractive power and provides high visual acuity. As mentioned earlier, when determining the degree of farsightedness, 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, obvious hypermetropia is determined, that is, the accommodation apparatus is maximally tense. If you use the first lens that provides normal visual acuity for spectacle correction, 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 selecting glasses, the refractive power of the lenses must be increased until the person begins to see the tenth row of the table blurry ( in this case, the refractive power of the lens will be minimal). After this, the lens is replaced with the one that preceded it, which will be used to make glasses.
Checking the sharpness binocular vision. Even if corrective lenses are selected correctly for each eye separately, it may turn out that after the glasses are made, objects visible through them will appear double. This deviation is usually due to impaired 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 selecting lenses, you need to check right in the ophthalmologist’s office whether the patient sees normally in both eyes ( There are many different tests for this).
Checking lens tolerance. After selecting corrective lenses, a person may experience certain discomfort In eyes ( tearing, stinging, burning), Related abrupt change state of accommodation systems. This is why, after fitting the lenses, the patient must remain in the trial frame for a few minutes. If after this no deviations are observed, you can safely write out a prescription for glasses.

When writing a prescription for glasses, the doctor must also indicate the distance between the centers of the pupils of both eyes of the patient. This parameter is determined using a millimeter ruler, and the distance is measured from the outer edge of the cornea of one eye to the inner edge of the cornea of the other eye. During measurements, the patient's eyes should be located directly opposite the doctor's eyes. When measuring the corneal edge on the right eye, the patient should look directly into the pupil of the doctor's left eye, and when measuring the corneal edge on the left eye, into the doctor's right pupil.

It is also worth noting that if you are farsighted, you should start wearing glasses as early as possible, as this will eliminate discomfort ( associated with blurred visual objects) and prevent the development of complications.

Does a child with farsightedness need glasses?

The need to wear glasses in children is determined by the cause and degree of farsightedness. So, for example, if farsightedness is physiological in nature, no correction is required, since the child’s vision independently normalizes by the age of 13–14 years. 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 farsightedness should be determined as soon as possible and glasses should be prescribed, since various complications develop in children much faster than in adults .

The selection of glasses for children follows the same rules as for adults. However, it is worth noting that as the child grows, the severity of hypermetropia may decrease ( due to the growth of the eyeball, increasing the refractive power of the cornea and lens). This is why children under 14 years of age are recommended to regularly ( semiannually) assess visual acuity, determine the degree of farsightedness and, if necessary, change the lenses in glasses.

Contact lenses for farsightedness

Principle of selection and appointment contact lenses the same as when prescribing 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 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 constantly changes ( when turning your eyes to the side, when glasses move away or approach). The contact lens is fixed directly on the cornea, as a result of which the distance from it to the retina remains constant. The lens also moves simultaneously with the eyeball, which helps to obtain 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 during rain, and do not fall out while bending your head, running, or during 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 practically invisible and do not cause any cosmetic inconvenience to a person, which cannot be said about glasses.

The disadvantages of contact lenses include the discomfort associated with their installation and removal, as well as the need to regularly change them ( The service life of even high-quality lenses does not exceed 1 month). Also, when using lenses, the risk of developing infectious complications (in case of non-compliance with personal hygiene rules).

Laser correction of farsightedness

Treatment of farsightedness using modern laser technologies allows, in some cases, to eliminate the existing vision defect, and to do this quite quickly, safely and painlessly.

Laser correction of farsightedness includes:

Photorefractive keratectomy ( PRK). The essence this method lies in the fact that with the help of a special laser removal ( evaporation) upper layer of the cornea ( stroma, which has refractive properties), resulting in changes ( intensifies) its refractive power. This allows you to reduce the degree of farsightedness and reduce the load on the accommodative system of the eye. The advantages of this method include safety and high efficiency ( for mild to moderate hypermetropia). The disadvantage of this method is that it takes a long time ( up to 1 month) recovery period and the possibility of corneal clouding in postoperative period, which is associated with damage to its upper ( epithelial) layer.
Transepithelial photorefractive keratectomy ( trans-FRK). The difference between this method and conventional PRK is that it causes 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), reduce 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 superficial epithelium and other tissues. This flap is raised, exposing the stroma itself. After this it is done laser removal stroma, necessary for normalizing the refractive system of the eye. Then the separated flap returns to its place, where it is almost instantly fixed 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 this, no stitches, scars or opacities remain on the cornea.

Lens replacement for farsightedness

Using this method, you can eliminate even severe farsightedness associated with damage to the lens ( including presbyopia). The essence of the method is that the old lens is removed from the eye, and a new one 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 a child's lens) it is possible to use special drugs that put the patient into medical sleep. In the latter case, the patient's length of stay in the hospital after surgery may 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 ( about 2 mm long) incision, after which, using a special ultrasonic device, it turns the lens into an emulsion ( liquid) and deletes it. Then an artificial lens is inserted into place of the lens, which itself straightens and is fixed in the desired position. Then the incision in the cornea 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, etc.).

Operations for farsightedness

Surgical treatment of farsightedness is indicated in cases where it is impossible to correct or eliminate this state other, less traumatic methods.

Surgical treatment of farsightedness includes:

Implantation of phakic lenses. The essence of the method is that a specially selected ( according to all the rules for selecting lenses for farsightedness) 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 normalizes). This eliminates a number of unpleasant aspects associated with the use of the latter ( in particular, the need for regular lens replacement disappears, since phakic lenses can last for many years). The disadvantages of the method include the fact that in case of progression of the disease and an increase in the degree of hypermetropia ( 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 ( moving from the pupil to the periphery) cuts. After fusion, these incisions change the shape of the cornea, that is, they increase its curvature, which leads to an increase in refractive power. It is worth noting that due to the long recovery period, the risk of corneal damage during surgery and frequent postoperative complications, this technique is practically not used today.
Keratoplasty. The essence of this method is the transplantation of a donor cornea, which was previously processed using special techniques ( that is, she was given special shape, providing the necessary refractive power). Donor cornea can be implanted ( implant) directly into the patient's cornea, attach to its outer surface or completely replace it.

Prevention of farsightedness

Prevention is a set of measures aimed at preventing the development of the 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, compliance with certain rules and recommendations will slow down the progression of the disease and reduce the likelihood of complications.

Prevention of farsightedness includes:

Timely and correct correction of farsightedness. This, perhaps, is the first and main measure to alleviate the course of the disease. Immediately after diagnosis, you should discuss with your doctor possible ways to eliminate the existing defect, and if this is not possible, choose optimal method corrections ( using glasses, contact lenses, etc.).
Elimination of excessive visual stress. For farsightedness ( without correction) there is constant tension in the ciliary muscle, which leads to an increase in the refractive power of the lens and allows, to a certain extent, to compensate for the existing defect. However, prolonged reading or working at a computer leads to accommodation fatigue, 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 that you regularly ( every 15 – 20 minutes) take a short break, during which you should move away from your workplace, walk around the house or do a few simple exercises for eyes.
Proper workplace lighting. As mentioned earlier, the development of visual discomfort, burning and pain in the eyes can be facilitated by working in poor lighting. That is why all people, and especially patients with farsightedness, should have proper lighting workplace. It is best to work in natural daylight, placing the table near a window. If you need to work in the dark, remember that direct light ( directed from the lamp directly to the workplace) has an extremely adverse effect on the eyes. It is best to use reflected light, for which you can point the lamp at a white ceiling or wall. Also, when working at a computer, it is recommended to turn on a lamp or a regular lamp ( that is, do not work in complete darkness), since the pronounced contrast between a bright monitor and a dark room significantly increases eye strain.
Regular visual acuity testing. Even after selecting corrective glasses or eliminating farsightedness using other methods, it is recommended regularly ( 1 – 2 times a year) visit an ophthalmologist. This will allow timely identification of various deviations ( for example, progression of presbyopia) and prescribe treatment in a timely manner.

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 down the progression of farsightedness, reducing the severity of clinical manifestations and preventing the development of complications.

A set of exercises for farsightedness includes:

Exercise 1. You should find the most distant point on the horizon ( house roof, tree 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 standing near a window or on the street. First, you should try to focus your vision on a nearby object ( for example, on the tip of the nose), and then look into the distance ( as far as possible), then repeat the procedure.
Exercise 3. If you get tired while reading, it is recommended to put the book down 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 eye muscles and also promotes temporary relaxation of accommodation.
Exercise 4. You need to close your eyes and slowly rotate your eyeballs clockwise and then in the opposite direction.

These exercises can be performed by both patients with farsightedness and healthy people. It is important to remember that you should start doing the exercises gradually, repeating them every 30 to 40 minutes ( when working at the computer or reading).

Complications of hypermetropia

As mentioned earlier, long-term progression of hypermetropia without appropriate correction can lead to a number of serious complications. Nonspecific complications of farsightedness include infection of the cornea ( keratitis), conjunctiva ( conjunctivitis), century ( blepharitis). This may be facilitated by impaired microcirculation in the structures of the eye associated with constant voltage accommodation and visual fatigue.

Farsightedness can also be complicated by:

spasm of accommodation;

Strabismus with farsightedness

It's called strabismus pathological condition, in which the pupils of both eyes “look” at various directions. With farsightedness, convergent strabismus can develop, in which the pupils of the eyes are excessively deviated towards the center. Reason for development this complication lies in the physiology of the visual analyzer. Under normal conditions, with tension in the accommodation apparatus ( that is, with an increase in the refractive power of the lenses) there is a natural convergence, that is, the bringing together of the pupils of both eyes. In a healthy person, this mechanism allows you to more accurately focus your gaze on a nearby object.

With severe farsightedness, there is a constant compensatory stress of accommodation ( that is, contraction of the ciliary muscle and increase in the refractive power of the lens), as a result of which convergence also occurs. Initially, this condition is easily eliminated by using farsightedness-correcting lenses. With long-term persistent tension of accommodation and accompanying convergence, an irreversible change in the extraocular muscles can occur, causing strabismus to become permanent ( what is most important in children).

Amblyopia ( lazy eye) with farsightedness

The essence of this disease consists in a decrease in visual acuity even with optimal correction of hypermetropia using lenses, and any other anatomical defects in the organ of vision cannot be identified. In other words, the lazy eye is functional impairment, which occurs with long-term progression of high degree hypermetropia.

With early detection and initiation of appropriate treatment, amblyopia can be eliminated ( treatment must be combined with adequate correction of farsightedness), however, the longer this condition persists, the more difficult it will be to restore normal function eyes later.

Accommodation spasm with farsightedness

The essence of this complication is a prolonged and pronounced contraction ( spasm) 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, an accommodation spasm can develop during prolonged work at the computer or while reading, that is, in the case when there is prolonged accommodation tension and overwork of the ciliary muscle. However, with severe farsightedness, accommodation is almost constantly tense, as a result of which the risk of developing a spasm increases significantly. That is why it is extremely important to begin correction and treatment of hypermetropia in a timely manner.

If a spasm of accommodation develops, it is recommended to interrupt the work you are doing and do several exercises to relax your eyes. If the spasm is severe, you should consult a doctor ( ophthalmologist). If necessary, the doctor can drop special drops into the patient’s eyes ( for example, atropine), as a result of which the opposite phenomenon will occur - the ciliary muscle will relax and fixate in this position for several hours or days, that is, paralysis of accommodation will occur.

Myopia with farsightedness

Myopia is a pathological condition in which a person has poor ( not clear) sees distant objects. Usually myopia develops as an independent disease ( what can be caused by poor visual hygiene?), and can also occur with long-term and uncorrected farsightedness.

The mechanism for the development of myopia is as follows. When focusing vision on a nearby object, the fibers of the ciliary muscle contract, the ligaments of the lens relax and its enlargement occurs ( lens) refractive power. When the vision moves to a more distant object, the ciliary muscle relaxes, the lens flattens, and its refractive power decreases. However, with a long, continuous stress of accommodation ( which is what is observed with uncorrected farsightedness) gradual hypertrophy occurs ( that is, an increase in size and strength) ciliary muscle. In this case, when accommodation relaxes, the muscle itself relaxes only partially, as a result of which the lens ligaments remain in a relaxed position, and the refractive power of the lens remains increased.

It is worth noting that the development of myopia with farsightedness 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, vision correction alone ( with glasses or contacts

Myopia makes life difficult for many modern people.

This pathology causes the eyeball to elongate, so light rays do not reach the retina and are focused in front of it. This causes a person to have poor distance vision.

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 myopia correction

To prevent myopia from developing and progressing, it is necessary to start using correction on time.

Correct selection of correction is the key to good vision

Important: At the initial stage, when the disease has not yet reached the limit of -1 D, it is not recommended to use the contact method.

Constant correction can cause deterioration.

If the patient has myopia with a deviation from the norm of -1 D or more, then the main way to stop the progression of the pathology is 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 children will not be able to use contact products independently.

The glasses are more suitable method improving vision in childhood myopia. Learn the selection rules in this article

There are the following rules for selecting contact correction for myopia:

It is best to choose products from silicone hydrogel.
Center thickness The therapeutic corrective attribute depends on the number of diopters required.
Product diameter must be suitable for the individual parameters of the patient’s eye. In order to determine this parameter, the ophthalmologist uses computer diagnostics organs of vision.
The healing attribute must be dispersive and have minus characteristics.
Selecting the right cylinder axes if myopia is complicated by astigmatism.
Choice wearing mode. These may be lenses that need to be removed at night and worn throughout the day. There is also the option of night lenses or permanent ones, which can be worn for 30 and more days without taking it off.
According to the nature of the design and shape you need to choose spherical. If astigmatism is present, then it will be suitable toric option. When presbyopia is noted, the doctor prescribes multifocal products.

Only an ophthalmologist can say for sure which lenses are best to choose for myopia.

Before selection, the ophthalmologist in mandatory conducts diagnostics and only based on the examination results does he make a final conclusion about the nature of the correction.

Features and benefits of using lenses for myopia

Medicine is actively developing. Today you can get rid of myopic disorder forever with laser surgery.

However, even despite this, lenses for correcting myopia remain relevant due to the following positive properties:

they do not limit the visual field;
they can be worn simultaneously with sunglasses;
ideal for active pastime;
no glare;
they do not fog up;
the image is not distorted;
they do not slide down like glasses;
have protective properties against ultraviolet radiation.

Those who choose this method of improving vision should also familiarize themselves with its features:

To put on the product, you need training and special skills;
addiction occurs gradually;
a healing attribute can slip out of your hands and get lost;
you need to learn how to properly care for and disinfect the product.

Fact: If hygiene and disinfection rules are not followed, complications may arise in the form of inflammatory processes.

If you use contact correction right, it will make life much easier and eliminate the inconvenience associated with poor eyesight.

Also check out the video 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 whose middle is thicker than their edges, and the group of diverging lenses includes 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 diverging lens.

Lenses are typically characterized by their optical power (measured in diopters), or focal length.

For building optical instruments with corrected optical aberration (primarily chromatic, caused by light dispersion - achromats and apochromats), other properties of lenses/their materials are also important, for example, refractive index, dispersion coefficient, transmittance of the material 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).

Types of lenses:
Collecting:
1 - biconvex
2 - flat-convex
3 - concave-convex (positive meniscus)
Scattering:
4 - biconcave
5 - flat-concave
6 - convex-concave (negative meniscus)

A 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 to correct dispersion or as a cover lens). Thus, the lenses of glasses for myopia are, as a rule, negative menisci.

A distinctive property 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.

Basic lens elements: NN - main optical axis - a straight line passing through the centers spherical surfaces, limiting the lens; O - optical center - the point that for biconvex or biconcave (with the same surface radii) lenses is located on the optical axis inside the lens (at its center).
Note. The path of the rays is shown as in an idealized (flat) lens, without indicating refraction at the real phase boundary. Additionally, a somewhat exaggerated image of a biconvex lens is shown

If a luminous point S is placed at a certain distance in front of the collecting lens, then a ray of light directed along the axis will pass through the lens without being refracted, and rays that do not pass 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 coming in a parallel beam, then upon exiting it the rays will refract at a larger angle and 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 incident on a diverging lens will be refracted towards the edges of the lens upon exiting it, that is, scattered. If these rays are continued in the opposite direction as shown in the figure with a dotted line, then they will converge at one point F, which will be focus this lens. This trick will imaginary.

Imaginary focus of a diverging lens

What has been said about focus on the main optical axis equally applies to those cases when the image of a point is on a secondary or inclined optical axis, that is, 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 at the conjugate focus - simply focal plane.

Collective lenses can be directed towards an object from either side, as a result of which rays passing through the lens can be collected from both one and the other side. 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.

Constructing an image with a thin converging lens

When presenting the characteristics of lenses, the principle of constructing an image of a luminous point at the focus of a lens was considered. Rays incident on the lens from the left pass through its rear focus, and rays incident on the right pass through its front focus. It should be noted that with diverging 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 a lens is obtained as follows: let’s say line AB represents an object located at a certain distance from the lens, significantly exceeding its focal length. From each point of the object, an innumerable number of rays will pass through the lens, of which, for clarity, the figure schematically shows the course of only three rays.

Three rays emanating from point A will pass through the lens and intersect at their respective vanishing points at A 1 B 1 to form an image. The resulting image is valid And upside down.

In this case, the image was obtained at a conjugate focus in a certain focal plane FF, somewhat distant from the main focal plane F’F’, running parallel to it through the main focus.

If an object is at an infinite distance from the lens, then its image is obtained at the rear focus of the lens F' valid, upside down And reduced until it looks like a point.

If an 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, upside down And reduced and will be located behind the main focus in the segment between it and the double focal length.

If an object is placed at double the focal length from the lens, then the resulting image is on the other side of the lens at double the focal length from it. The image is obtained valid, upside down And equal in size subject.

If an object is placed between the front focus and double focal length, then the image will be obtained behind double focal length and will be valid, upside down And enlarged.

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 only be obtained at infinity.

If an 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 intersecting anywhere. The image is then imaginary, direct And enlarged, i.e. in this case the lens works like a magnifying glass.

It is easy to notice that when an object approaches the front focus of the lens from infinity, the image moves away from the back focus and, when the object reaches the front focus plane, it appears at infinity from it.

This pattern has great importance in practice various 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, you need to know the basic lens formula.

Thin Lens Formula

The distances from the object point to the center of the lens and from the image point to the center of the lens are called conjugate focal lengths.

These quantities are interdependent 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; - the main focal length of the lens. In the case of a thick lens, the formula remains unchanged with the only difference being that the distances are measured not from the center of the lens, but from the main planes.

To find one or another unknown quantity with two known ones, use the following equations:

It should be noted that the signs of the quantities u , v , f are selected based on the following considerations - for a real image from a real object in a converging lens - all these quantities 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 diverging, the focal length is negative.

Image scale

The image scale () is the ratio of the linear dimensions of the image to the corresponding linear dimensions of the object. This relationship can be indirectly expressed by the fraction , where is the distance from the lens to the image; - distance from the lens to the object.

There is a reduction factor here, i.e. a number showing how many times the linear dimensions of the image are smaller than the actual linear dimensions of the object.

In the practice of calculations, it is much more convenient to express this relationship in values or , where is the focal length of the lens.

Calculation of focal length and optical power of a lens

The lenses are symmetrical, that is, they have the same focal length regardless of the direction of 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 facing the light.

Combination of multiple lenses (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 the simple sum of the optical powers of each lens (assuming 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 a certain 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 main planes of the lenses.

Disadvantages of a simple lens

Modern photographic equipment places high demands on image quality.

The image produced by a simple lens, due to a number of shortcomings, does not satisfy these requirements. Elimination of most of the shortcomings is achieved by appropriate selection of a number of lenses into a centered optical system - lens. Images obtained with simple lenses have various disadvantages. Disadvantages of optical systems are called aberrations, which are divided into the following types:

Geometric aberrations
Diffraction 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 remelted pure silica with minor (about 0.01%) additions of Al 2 O 3, CaO and MgO. It is characterized by high heat resistance and inertness to many chemicals with the exception of hydrofluoric acid.