home Audience 12 Lenses: types of lenses (physics). Types of collecting, optical, diverging lenses. How to determine the type of lens? Organic polymer lenses

Lenses: types of lenses (physics). Types of collecting, optical, diverging lenses. How to determine the type of lens? Organic polymer lenses

GAPOU "Akbulak Polytechnic College"
Lesson plan for the discipline: PHYSICS
Lesson No. 150
Cattle
date group
Topic of the lesson: Lenses. Thin Lens Formula
Lesson objectives:
Educational –
` formulate the concept of a lens, what types of lenses there are;
` show the main characteristic points of the lens (optical center, main optical axis, main focal points of the lens)
` in weight the basic formulas of a thin lens
Developmental – to promote the development of: thinking, spatial imagination, communication skills; continue the formation of a scientific worldview;
Educational – To develop a culture of mental work and a naturally materialistic worldview, through lessons to instill interest in physics as a science.
. Type of lesson:_ theoretical
Equipment Laptop, projector, electronic textbook
LESSON CONTENT
No. Stages of the lesson, questions of the lesson Forms and methods of teaching Time regulations
1 Organizational stage:
Checking Attendance
Checking students' readiness for class
Checking homework Establishing the class's readiness for the lesson. 2-3 min.
2 Message about the topic of the lesson Slides, blackboard 2 min.
3 Motivational point:
Justification of the need to study this topic for effective mastery of physics
In previous lessons, we studied how light behaves under different conditions. We studied the laws of optics. How do you think people use these laws for any practical purposes?
Involving students in the process of setting goals and objectives for the lesson
Conversation. Activity analysis 2-3 min
4 Updating basic knowledge:
What topic did you start studying?
What laws have you become familiar with?
Formulate the law of rectilinearity of light propagation.
Formulate the law of light reflection.
Formulate the law of light refraction. Frontal conversation 5-7 min.
5. Work on the topic of the lesson:
What is a lens? What types of lenses are there?
The first mention of lenses can be found in an ancient Greek play
Aristophanes "Clouds" (424 BC), where with the help of a convex
glass and sunlight produced fire.
Lens from him. linse, from Latin lens - lentilTypes of lenses
Basic lens elements
MAIN OPTICAL AXIS is a straight line passing through
the centers of the spherical surfaces delimiting the lens.
OPTICAL CENTER - the intersection of the main optical axis with the lens, indicated by point O.
A secondary optical axis is any straight line passing through the optical center.
If a beam of rays falls on a collecting lens,
parallel to the main optical axis, then after
refraction in the lens they are collected at one point F,
which is called the main focus of the lens.
There are two main focuses; they are located on the main optical axis at the same distance from the optical center of the lens on opposite sides.
Thin lens - a lens whose thickness is small compared to the radii of curvature of the spherical surfaces limiting it.
Thin Lens Formulas
Lens power
1 diopter is the optical power of a lens whose focal length is 1 meter.
Images produced by the lens
Types of images
Constructing images in a converging lens
Legend
F – lens focus
d - distance from object to lens
f – distance from the lens to the image
h – object height
H – image height
D - Optical power of the lens.
Units of optical power - diopter - [dtpr]
G – lens magnification
Practical significance of the topic being studied Working with ICT
Electronic textbook 22-28 min
6 Summing up the lesson, evaluating the results of the work Conversation 2-3 min
7. Homework 18.4. 331-334 p. 1-2 min
8. Reflection: to what extent have the goals and objectives of the lesson been achieved? Conversation 1-2 min
Teacher: G.A.Krivosheeva

Barabinsk branch of the Novosibirsk College of Transport Technologies named after N.A. Lunina.

Teacher: Nagoga Ekaterina Mikhailovna.

Topic: “Lenses. Construction in lenses. Thin lens formula."

Target: provide knowledge about lenses, their physical properties and characteristics.

During the classes

Organizing time

Greetings.

Checking homework.

II. Learning new material

The phenomenon of light refraction underlies the action of lenses and many optical instruments used to control light beams and obtain optical images.

Lens is an optical transparent body bounded by spherical surfaces. Existstwo types of lenses :

a) convex;

b) concave.

There are convex lenses : biconvex, plano-convex, concave-convex.

Concave lenses can be : biconcave, plano-concave, convex-concave.

Lenses whose centers are thicker than their edges are calledcollecting , and which have thicker edges- scattering (slides 3,4) .

Experiment

A beam of light is directed onto a biconvex lens. We are watchingthe collecting effect of such a lens: each ray incident on the lens, after being refracted by it, deviates from its original direction, approaching the main optical axis.

The experience described naturally leads students to the concepts of principal focus and focal length of a lens.

The distance from the optical center of the lens to its main focus is calledfocal length of the lens . It is designated by the letterF, like the trick itself (slides 4-6).

Next, the path of light rays through a diverging lens is determined. The question of the action and parameters of a diverging lens is considered in a similar way. Based on experimental data, we can conclude: the focus of the diverging lens is imaginary (slide 7).

III . Construction in lenses.

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.

(slides 8,9)

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.

(slide 10)

If an object is placed between the front focus and double focal length, the image will be obtained behind double focal length and will be real, inverted and enlarged.

(slide 11)

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 real, upside down and equal in size to the object.

(slide 12)

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

(slide 13)

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.

(slide 14)

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 thenimaginary , direct And enlarged , i.e. in this case the lens works like a magnifying glass.

(slide 15)

IV. Derivation of the thin lens formula.

(slide 16)

From the similarity of the shaded triangles (Fig. 70) it follows:

(slide 17)

Whered - distance of the object from the lens;fdistance from lens to image;F - focal length. The optical power of the lens is:

When calculating, the numerical values of real quantities are always substituted with a “plus” sign, and imaginary ones with a “minus” sign (slide 18).

Linear increase

From the similarity of the shaded triangles (Fig. 71) it follows:

(slide 19)

V. Consolidation of the studied material.

Why is the focus of a diverging lens called imaginary?

How does a real image of a point differ from an imaginary one?

By what sign can you tell whether this lens is converging or diverging, judging only by its shape?

State the property of a convex lens.(Collect parallel rays into one point.)

Solving problems No. 1064, 1066 (P) (slides 20,21)

§ 63-65, No. 1065(R)

Plan:

1. History
2 Characteristics of simple lenses
3 Path of rays in a thin lens
4 Ray path in the lens system
5 Constructing an image with a thin converging lens
6 Thin Lens Formula
7 Image scale
8 Calculation of focal length and optical power of a lens
9 Combination of multiple lenses (centered system)
10 Disadvantages of a simple lens
11 Lenses with special properties
- 11.1 Organic polymer lenses
- 11.2 Quartz lenses
- 11.3 Silicon lenses
12 Use of lenses

Introduction

Plano-convex lens

Lens(German) Linse, from lat. lens- lentil) - a part made of an optically transparent homogeneous material, limited by two polished refractive surfaces of rotation, for example, spherical or flat and spherical. Currently, “aspherical lenses”, the surface shape of which differs from a sphere, are increasingly being used. Optical materials such as glass, optical glass, optically transparent plastics and other materials are commonly used as lens materials.

Lenses are also called other optical devices and phenomena that create a similar optical effect without having the specified external characteristics. For example:

Flat “lenses” made of a material with a variable refractive index that changes depending on the distance from the center
Fresnel lenses
Fresnel zone plate using diffraction phenomenon
“lenses” of air in the atmosphere - heterogeneity of properties, in particular, the refractive index (manifested in the form of flickering images of stars in the night sky).
Gravitational lensing is the effect of deflection of electromagnetic waves by massive objects observed at intergalactic distances.
A magnetic lens is a device that uses a constant magnetic field to focus a beam of charged particles (ions or electrons) and is used in electron and ion microscopes.
The image of a lens formed by an optical system or part of an optical system. Used in the calculation of complex optical systems.

1. History

First mention of lenses can be found in the ancient Greek play "The Clouds" by Aristophanes (424 BC), where fire was produced using convex glass and sunlight.

From the works of Pliny the Elder (23 - 79) it follows that this method of kindling fire was also known in the Roman Empire - it also describes, perhaps, the first case of using lenses to correct vision - it is known that Nero watched gladiatorial fights through a concave emerald to correct myopia .

Seneca (3 BC - 65) described the magnifying effect that a glass ball filled with water gives.

The Arab mathematician Alhazen (965-1038) wrote the first significant treatise on optics, describing how the lens of the eye creates an image on the retina. Lenses only came into widespread use with the advent of glasses around the 1280s in Italy.

The Golden Gate is visible through raindrops acting as lenses.

Plant seen through a biconvex lens

2. Characteristics of simple lenses

Depending on the forms there are collecting(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 the surrounding medium. 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.

To build optical devices 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), diffuse (thickens towards the edges) or telescopic (focal length is infinity). So, for example, the lenses of glasses for myopia are, as a rule, negative menisci.

Contrary to popular misconception, the optical power of a meniscus with equal radii is not zero, but positive, and depends on the refractive index of the glass and the thickness of the lens. A meniscus, the centers of curvature of the surfaces of which are located at one point, is called a concentric lens (optical power is always negative).

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.

The main elements of the lens: NN - optical axis - a straight line passing through the centers of the spherical surfaces delimiting 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 rays is shown as in an idealized (thin) 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 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 the 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 focus F’, and the distance from the center of the lens to the focus is the 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 optical axis equally applies to those cases when the image of a point is on an inclined line passing through the center of the lens at an angle to the optical axis. The plane perpendicular to the optical axis, located at the focus of the lens, is called 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 main points of the lens.

3. Path of rays in a thin lens

A lens for which the thickness is assumed to be zero is called “thin” in optics. For such a lens, they show not two main planes, but one in which the front and back seem to merge together.

Let us consider the construction of a beam path of an arbitrary direction in a thin collecting lens. To do this, we use two properties of a thin lens:

The beam passing through the optical center of the lens does not change its direction;

Parallel rays passing through the lens converge at the focal plane.

Let us consider a ray SA of an arbitrary direction incident on a lens at point A. Let us construct a line of its propagation after refraction in the lens. To do this, we construct a ray OB parallel to SA and passing through the optical center O of the lens. According to the first property of the lens, the ray OB will not change its direction and will intersect the focal plane at point B. According to the second property of the lens, the parallel ray SA after refraction must intersect the focal plane at the same point. Thus, after passing through the lens, the ray SA will follow the path AB.

Other beams, such as the SPQ beam, can be constructed in a similar way.

Let us denote the distance SO from the lens to the light source by u, the distance OD from the lens to the point of focusing of the rays by v, and the focal length OF by f. Let us derive a formula connecting these quantities.

Let's consider two pairs of similar triangles: 1) SOA and OFB; 2) DOA and DFB. Let's write down the proportions

Dividing the first proportion by the second, we get

After dividing both sides of the expression by v and rearranging the terms, we arrive at the final formula

where is the focal length of the thin lens.

4. Ray path in the lens system

The path of rays in a lens system is constructed using the same methods as for a single lens.

Consider a system of two lenses, one of which has a focal length OF, and the second O 2 F 2. We construct the path SAB for the first lens and continue the segment AB until it enters the second lens at point C.

From point O 2 we construct a ray O 2 E, parallel to AB. When intersecting the focal plane of the second lens, this ray will give point E. According to the second property of a thin lens, ray AB, after passing through the second lens, will follow the path BE. The intersection of this line with the optical axis of the second lens will give point D, where all the rays emerging from the source S and passing through both lenses will be focused.

5. Constructing an image with a thin collecting 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 is of great importance in the practice of various types of 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.

6. 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 thin lens formula(discovered by Isaac Barrow):

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.

Images of black letters through a thin convex lens with focal length f (displayed in red). Shown are the rays for the letters E, I, and K (in blue, green, and orange, respectively). The dimensions of the real and inverted images E (2f) are the same. Image I (f) - at infinity. K (at f/2) has double the size of the virtual and direct image

7. 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.

8. Calculation of focal length and optical power of the lens

The focal length value for a lens can be calculated using the following formula:

, Where

Refractive index of the lens material,

The distance between the spherical surfaces of a lens along the optical axis, also known as lens thickness, and the signs of the radii are considered positive if the center of the spherical surface lies to the right of the lens and negative if to the left. If it is negligibly small relative to its focal length, then such a lens is called thin, and its focal length can be found as:

where R>0 if the center of curvature is to the right of the main optical axis; R<0 если центр кривизны находится слева от главной оптической оси. Например, для двояковыпуклой линзы будет выполняться условие 1/F=(n-1)(1/R1+1/R2)

(This formula is also called thin lens formula.) The focal length is positive for converging lenses, and negative for diverging ones. The quantity is called optical power lenses. The optical power of a lens is measured in dioptres, the units of which are m −1 .

These formulas can be obtained by carefully considering the process of constructing an image in a lens using Snell's law, if we move from general trigonometric formulas to the paraxial approximation.

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.

9. 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.

10. 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 shortcomings is achieved by appropriate selection of a number of lenses into a centered optical system - a 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
- Spherical aberration;
- Coma;
- Astigmatism;
- Distortion;
- Image field curvature;
Chromatic aberration;
Diffraction aberration (this aberration is caused by other elements of the optical system and has nothing to do with the lens itself).

11. Lenses with special properties

11.1. Organic polymer lenses

Polymers make it possible to create inexpensive aspherical lenses using casting.

Contact lenses

In the field of ophthalmology, soft contact lenses have been developed. Their production is based on the use of materials of a biphasic nature, combining fragments organosilicon or organosilicon polymer silicone and a hydrophilic hydrogel polymer. Work over more than 20 years led to the creation in the late 90s of silicone hydrogel lenses, which, thanks to the combination of hydrophilic properties and high oxygen permeability, can be used continuously for 30 days around the clock.

11.2. 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.

Transparent quartz glass transmits ultraviolet and visible light rays well.

11.3. Silicon lenses

Silicon combines ultra-high dispersion with the highest absolute value of refractive index n=3.4 in the IR range and complete opacity in the visible range of the spectrum.

In addition, it was the properties of silicon and the latest technologies for its processing that made it possible to create lenses for the X-ray range of electromagnetic waves.

12. Use of lenses

Lenses are a universal optical element of most optical systems.

The traditional use of lenses is binoculars, telescopes, optical sights, theodolites, microscopes and photographic and video equipment. Single converging lenses are used as magnifying glasses.

Another important area of application of lenses is ophthalmology, where without them it is impossible to correct vision defects - myopia, farsightedness, improper accommodation, astigmatism and other diseases. Lenses are used in devices such as glasses and contact lenses.

In radio astronomy and radar, dielectric lenses are often used to collect a flux of radio waves into a receiving antenna or focus them on a target.

In the design of plutonium nuclear bombs, lens systems made of explosives with different detonation speeds (that is, with different refractive indexes) were used to convert a spherical diverging shock wave from a point source (detonator) into a spherical converging one.

Notes

Science in Siberia - www.nsc.ru/HBC/hbc.phtml?15 320 1
silicon lenses for the IR range - www.optotl.ru/mat/Si#2

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This abstract is based on an article from Russian Wikipedia. Synchronization completed 07/09/11 20:53:22
Related abstracts: Fresnel lens, Luneberg lens, Billet lens, Electromagnetic lens, Quadrupole lens, Aspheric lens.

The lens represents a body, transparent and limited. The limiters of the lens body are most often either two curved surfaces, or one curved and the other flat. As you know, lenses can be convex or concave. Accordingly, a lens whose middle plane is thickened relative to its edges is convex. Concave lenses present a different picture: their middle is thinner relative to the edge surface. If the refractive index of the rays of the environment is less than the same index of a convex lens, then in it the beam formed by parallel rays is refracted and transformed into a converging beam. Concave lenses with such properties are called converging lenses. If in a concave lens a beam of parallel directed rays turns into divergent upon refraction, then these are divergent concave lenses; in them, air acts as the external medium.

The lens is a spherical surface with geometric centers. The straight line that connects the centers is the main optical axis. Thin lenses have a thickness less than their radius of curvature. For such lenses, it is true that their segment vertices are closely spaced and represent an optical center. In this case, a secondary axis is any straight line passing through the center at an angle to the straight line connecting the centers of the spherical surfaces. But to determine the main focus of a lens, it is enough to imagine that a beam of rays hits a collecting concave lens. Moreover, these rays are parallel to the main axis. After refraction, such rays will gather at one point, which will be the focus. In focus you can see the continuation of the rays. These are rays directed parallel to the main axis before refraction. But this trick is imaginary. There is also a main focus of the diverging lens. Or rather, two main focuses. If you imagine the main optical axis, then the main foci will be on it at an equal distance from the center. If we calculate the reciprocal of the focal length, we get the optical power.

The unit of optical power of a lens is the diopter, if we mean the SI system. Typically, for a converging lens, its optical power is positive, while for a diverging lens it will be negative. If the plane has the property of passing through the main focus of the lens and at the same time perpendicular to the main axis, then it is the focal plane. It is reliably known that rays in the form of a beam directed at the lens and at the same time being parallel to the secondary optical axis will be collected at the intersection of the axis and the focal plane. The ability of lenses to reflect and refract is used in optical instrumentation.

We all know examples of everyday use of lenses: a magnifying glass, glasses, a camera, in science and research it is a microscope. The significance of the discovery of the properties of lenses for humans is enormous. In optics, spherical lenses are most often used. They are made of glass and limited to spheres.

Types of lenses Thin - the thickness of the lens is small compared to the radii of the lens surfaces and the distance of the object from the lens. Thin lens formula 1 1 + 1 = F d f . F= d f ; d+ f where F – focal length; d is the distance from the object to the lens; f – distance from the lens to the image optical center R 1 О О 1 main optical axis R 2 О 2

Characteristics of lenses 1. Focal length The point at which the rays intersect after refraction in the lens is called the main focus of the lens (F). F

Characteristics of lenses 1. Focal length A converging lens has two main actual foci. F Focal length (F)

Characteristics of lenses 2. Optical power of a lens The reciprocal of the focal length is called the optical power of the lens D = 1/F Measured in diopters (dopters) 1 diopter = 1/m The optical power of a converging lens is considered a positive value, and a diverging lens is considered a negative value.

Protecting your vision You must: You must not: • look at an object; read while eating, by candlelight, in a moving vehicle and lying down; a distance of at least 30 cm, sit at the computer at a distance of 6070 cm from the screen, from the TV - 3 m (the screen should be at eye level); Ш so that the light falls from the left side; Ш skillfully use household appliances; Ш types of work that are dangerous to the eyes should be performed with special glasses; § watch TV continuously for more than 2 hours; § so that the room lighting is too bright; § openly look at direct rays of sunlight; § Rub your eyes with your hands if you get dust. If a foreign body gets into your eye, wipe your eye with a clean, damp cloth. If you notice problems with your vision, consult a doctor (ophthalmologist).