What point is called the focus of the lens. Thin lenses

main focus

in optics, the point at which, after passing through an optical system, a beam of light rays incident on the system parallel to its optical axis converges. In the case when a beam of parallel rays as a result of passing through optical system diverges, G. f. is the point of intersection of the lines that serve as continuations of the rays leaving the system. On the contrary, a beam of rays emanating from the focus, as a result of passing through the optical system, turns into a beam of rays parallel to the axis of the system. Distinguish front G. f., corresponding to a beam of parallel rays leaving the system, and rear G. f., corresponding to a beam of parallel rays entering the system (see. rice. ). Both G. f. lie on the optical axis of the system.

In astronomy G. f. often referred to as the surface in which the main mirror Reflector a or the lens Refractor a builds an image of the observed area of ​​the celestial sphere. For correction of a coma (See. Coma) and increase in the field of good images in a reflector before G. f. a lens corrector (for example, a Ross lens) is placed. In the largest reflectors in G. f. the cabin for the observer, which is called the cabin of the main focus, is being strengthened.

A parallel beam of rays incident on the system is collected at the rear main focus F"; rays coming from the front focus F exit the system in a parallel beam.

Great Soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .

See what "Main Focus" is in other dictionaries:

The main focus is the point at which, after passing through the optical system, a beam of light rays converges, incident on the system parallel to its optical axis. In the case when a beam of parallel rays as a result of passing through an optical ... ... Wikipedia

In optics, (see CARDINAL POINTS OF THE OPTICAL SYSTEM). Physical encyclopedic Dictionary. Moscow: Soviet Encyclopedia. Chief Editor A. M. Prokhorov. 1983... Physical Encyclopedia

1. FOCUS, a; m. [it. Focus from lat. focus focus] 1. Phys. The point at which, after a parallel beam of rays has passed through an optical system, the latter intersect. F. lenses. F. eye lens. Short f. (distance from the refractive or ... ... encyclopedic Dictionary

This term has other meanings, see Focus. Focus (from Latin focus "center") of the optical system is the point at which initially parallel light rays intersect ("focus") after passing through the collecting optical ... ... Wikipedia

Focus (from Latin focus "fire") of an optical system is the point at which initially parallel light rays intersect ("focus") after passing through a collecting optical system (or where their continuations intersect, if the system ... ... Wikipedia

- (from lat. focus hearth, fire) in optics, a point, in a swarm after passing through a parallel beam of optical rays. beams of the system intersect (or their extensions, if the system transforms a parallel beam into a divergent one). If the rays pass ... ... Physical Encyclopedia

The point of application of the lift increment (∆)Y when changing the angle of attack (α). In F. a. the longitudinal moment coefficient tz does not depend on the angle of attack or the lift coefficient cy (see Aerodynamic coefficients). The concept of F. a. applicable to... ... Encyclopedia of technology

This term has other meanings, see Focus ... Wikipedia

This term has other meanings, see Hocus pocus. Hocus pocus

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What is the focus of a lens? If a beam of rays parallel to the main optical axis falls on a converging lens, then after refraction in the lens they are collected at one point F, which is called the main focus of the lens.

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lenses

"Physicist's camera" - 2. 6. Lens - system optical lenses enclosed in a special frame. -) Snapshot of Talbot. The main characteristics of the lens: -) Daguerre shot. Relationship between lens-to-subject distance and lens-to-image distance. Reference abstract on the topic "Camera". Photography (Greek) - drawing with light, light painting.

"Lenses" - Eye. Basic elements of a lens. Chromatic aberration -. Deficiencies in human vision. In the figure it is indicated as follows: - collecting - scattering. Prepared by: physics teacher of the 1st category Kolomiets I.M. An example of constructing an image of an arbitrary object. Content.

"The Lens Lesson" - Concave Lenses. Building an image in a lens. Physics lesson on the topic “Lens. The reciprocal of the focal length is called optical power lenses. Lesson objective: Survey homework: What is a lens? 1a 2c 3a 4c 5b 6c 7a. Side optical axis. Divergent lens. The optical power of the lens.

“Constructing an image in a lens” - “Constructing an image in lenses”. Show the course of rays in a converging lens. Real Inverted Diminished. Real Inverted Magnified. Lesson objectives: Conclusion: Building images in a converging lens. Construct the further course of the beam in the prism.

"Optical power of the lens" - The optical power of the lens. Lenses. What kind of lenses are available? I option. What is a lens? Lens from him. linse, from lat.lens - lentils. Optical devices. Types of lenses. Image: real, inverted, enlarged. Side optical axis. Gathering. Build an image of the object proposed in the figure.

"Lens" - Each lens has two focal points - one on each side. Biconvex (1) Plano-convex (2) Concave-convex (3). Basic designations in the lens. If the object is in double focus, then the image will be real, equal, reverse. If the subject is in focus, then there is no image. If the object is between the focus and the optical center, then the image is imaginary, direct, enlarged.

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Focal length- physical characteristic of the optical system. For a centered optical system consisting of spherical surfaces, describes the ability to collect rays into one point, provided that these rays come from infinity in a parallel beam parallel to the optical axis.

For a lens system, as well as for a simple lens of finite thickness, the focal length depends on the radii of curvature of the surfaces, the refractive indices of the glasses and the thicknesses.

Defined as the distance from the front principal point to the front focus (for front focal length), and as the distance from the back principal point to the back focus (for back focal length). In this case, the main points are the points of intersection of the front (rear) main plane with the soptic axis.

The value of the back focal length is the main parameter that is used to characterize any optical system.

A parabola (or paraboloid of revolution) focuses a parallel beam of rays into one point

Focus(from lat. focus- "center") of an optical (or operating with other types of radiation) system - the point at which intersect ( "focused") initially parallel rays after passing through the collecting system (or where their continuations intersect, if the system is scattering). The set of foci of the system defines its focal surface. The main focus of the system is the intersection of its main optical axis and the focal surface. Currently, instead of the term main focus(front or back) terms are used back focus and front focus.

optical power- value characterizing the refractive power of axisymmetric lenses and centered optical systems of such lenses. The optical power is measured in diopters (in SI): 1 diopter \u003d 1 m -1.

Inversely proportional to the focal length of the system:

where is the focal length of the lens.

The optical power is positive for collecting systems and negative for scattering systems.

The optical power of a system consisting of two lenses in the air with optical powers and is determined by the formula:

where is the distance between the rear main plane of the first lens and the front main plane of the second lens. In the case of thin lenses, it coincides with the distance between the lenses.

Typically, optical power is used to characterize lenses used in ophthalmology, in spectacle designations, and for a simplified geometric definition of the beam path.

To measure the optical power of lenses, dioptrimeters are used, which allow measurements, including astigmatic and contact lenses.

18. The formula for conjugate focal lengths. Building an image with a lens.

Conjugate focal length- the distance from the rear main plane of the lens to the image of the object, when the object is located not at infinity, but at some distance from the lens. The conjugate focal length is always greater than the focal length of the lens and the greater the less distance from the object to the front principal plane of the lens. This dependence is shown in the table, in which the distances and are expressed in quantities.

For a lens, these distances are related by the ratio that follows directly from the lens formula:

or, if d and R are expressed in terms of focal length:

b) Image construction in lenses.

To construct the path of a beam in a lens, the same laws apply as for a concave mirror. Ray, axis parallel, passes through the focus and vice versa. The central beam (the beam going through the optical center of the lens) passes through the lens no deviation; in thick

in lenses, it shifts slightly parallel to itself (as in a plane-parallel plate, see Fig. 214). It follows from the reversibility of the path of the rays that each lens has two foci that are at the same distance from the lens (the latter is true only for thin lenses). For thin converging lenses and central rays, the following are true: imaging laws:

g > 2F; image reverse, reduced, real, b > F(Fig. 221).

g = 2F; image inverse, equal, real, b = F.

F < g < 2F; image reverse, enlarged, real, b > 2F.

g < F; the image is direct, enlarged, imaginary, - b > F.

At g < F the rays diverge, intersect on continuation and give an imaginary

image. The lens acts like a magnifying glass (loupe).

Images in diverging lenses are always imaginary, straight and reduced (Fig. 223).

1. Transparent body with two spherical surfaces.

2. What is the difference between convex and concave lenses?

2. 1) The middle is thicker than the edges.

2) The middle is thinner than the edges.

3. Which lenses are converging and which are diverging?

3. 1) Converting a parallel beam of rays into a convergent one;

2) divergent.

4. What is called the main optical axis of the lens?

5. What point is called the main focus of the lens?

5. The point at which rays or their extensions intersect after refraction.

6. What is the focal length of the lens?

6. Distance from the optical center to the main focus.

7. What is called the optical power of the lens?

7. Physical quantity, the reciprocal of the focal length.

8. What is the name of the unit of optical power of a lens?

8. Diopter.

9. How can you measure the focal length of a converging lens?

9. Having directed the sun's rays to the lens, measure the distance from it to the image of the Sun, where it will be clear.

10. Which lenses have a positive optical power, which have a negative one?

10. For those who gather it is positive, for those who scatter it is negative.

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The main focus of a lens is the point at which parallel rays of light incident on the lens converge.

The distance from the main focus of a lens to its optical center is called the focal length of the lens. Each lens has two foci, as it can refract light rays from both sides. The foci are numbered (first, second) in the direction of the rays incident on the lens.

The distance F between the main focus of a lens and its optical center is called the main focal length. If the main focus is real, then F is considered positive, and if it is imaginary, negative.

The object is located between the double and main focus of the lens.

The plane passing through the main focus of the lens perpendicular to the main optical axis is called the focal plane.

The plane passing through the main focus of the lens perpendicular to its main optical axis is called the focal plane.

The plane passing through the main focus of the lens perpendicular to the main optical axis is called the focal plane.

Recall briefly that the main focus of the lens is the point where all the rays converge, going before refraction parallel to the optical axis. A biconvex lens has two main foci located on either side of the lens. The back focus F t lies in image space.

Since the scale is at the main focus of the lens, the rays from any division of the scale exit the lens in parallel; if the telescope is adjusted to observe celestial objects, then the scale will optically coincide with the telescope's cross. If this division of the scale coincides with the center of the telescope's cross, the line connecting this division with the optical center of the lens should be parallel to the telescope's line of sight. By fixing the magnet and moving the telescope, we can set the angular value of the scale divisions, and then, when the magnet is suspended and the position of the telescope is known, we can determine the position of the magnet at any time by reading the reading from the scale division that coincides with the cross.