Lens, optical power of the lens

Refraction of light is widely used in various optical instruments: cameras, binoculars, telescopes, microscopes. The lens is an indispensable and most essential part of such devices. And the optical power of the lens is one of the main magnitudes that characterizes any optical device.

An optical lens or optical glass is a light-permeable glass body that is bounded on both sides by spherical or other curved surfaces (one of the two surfaces can be flat).

In the form of confining surfaces, they can be spherical, cylindrical, and others. Lenses that are mid-thicker than edges are called convex; With the edges thicker than the middle - concave.
If we put a parallel beam of light rays onto a convex lens, and then place the screen behind it, then, moving it relative to the lens, we get a small bright spot on it. This it, breaking the rays falling on it, collects them. Therefore, it is called collector. A concave lens, refracting light, dispels it to the sides. It is called dispersive.

The center of the lens is called its optical center. Any straight line that passes through it is called the optical axis. And the axis that intersects the central points of spherical refractive surfaces, was called the main (main) optical axis of the lens, others - the side axes.

If we direct an axial beam parallel to its axis to the collecting lens , then passing this beam will intersect the axis at a certain distance from it. This distance is called the focal length, and the point of intersection is its focus. All lenses have two foci, which are on both sides. Based on the laws of light refraction, one can theoretically prove that all axial rays, or rays traveling near the main optical axis, falling on a thin collective lens parallel to its axis, converge in focus. Experience confirms this theoretical proof.

By letting a beam of axial rays parallel to the main optical axis to a thin biconvex lens, we find that out of it these beams emerge as a beam that diverges. If such a divergent beam hits our eye, it will seem to us that the rays come out of one point. This point is called the imaginary focus. The plane, which is perpendicular to the main optical axis through the focus of the lens, is called the focal plane. The focal planes of the lens are two, and they are on either side of it. When a beam of rays is directed onto the lens, which are parallel to any of the side optical axes, this beam, after it has been refracted, converges on the corresponding axis at the point of its intersection with the focal plane.

The optical force of a lens is a value that is the opposite of its focal length. We define it using the formula:
1 / F = D.

The unit of measurement of this force is called diopter.
1 dioptry is the optical power of a lens having a focal length of 1 m.
For convex lenses this force is positive, and for concave lenses it is negative.
For example: What is the optical power of a spectacled convex lens, if F = 50 cm is its focal length?
D = 1 / F; By the condition: F = 0.5 m; Hence: D = 1 / 0.5 = 2 diopters.
The magnitude of the focal length, and, consequently, the optical power of the lens is determined by the refractive index of the substance from which the lens consists and by the radius of the spherical surfaces bounding it.

The theory gives a formula by which it can be calculated:
D = 1 / F = (n-1) (1 / R1 + 1 / R2).
In this formula, n is the refraction of the lens material, R1, 2 is the radius of curvature of the surface. Radii of convex surfaces are considered positive, and concave - negative.

The character of the object image obtained from the lens, that is, its magnitude and position, depends on the location of the object with respect to the lens. The location of the object and its magnitude can be found using the lens formula:
1 / F = 1 / d + 1 / f.
To determine the linear magnification of a lens, we use the formula:
K = f / d.

The optical power of a lens is a concept that requires a detailed study.