Refraction of light is widely used in various optical devices: cameras, binoculars, telescopes, microscopes. An indispensable and most essential part of such devices is the lens. And the optical power of the lens is one of the main quantities 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 may be flat).
In the form of bounding surfaces, they can be spherical, cylindrical, and others. Lenses that have a middle thicker than the edges are called convex; with edges thicker than the middle - concave.
If we let a parallel beam of light beams onto a convex lens and place a screen behind it, then moving it relative to the lens, we get a small bright spot on it. It is she who, refracting the rays incident on her, collects them. Therefore, it is called collecting. A concave lens that refracts light scatters it to the sides. It is called scattering.
The center of the lens is called its optical center. Any line that passes through it is called the optical axis. And the axis crossing the central points of the spherical refracted surfaces is called the main (main) optical axis of the lens, others - the side axes.
If an axial beam parallel to its axis is directed to a collecting lens , then passing it, this beam will cross the axis at a certain distance from it. This distance is called the focal length, and the intersection point itself is called its focus. All lenses have two focuses, which are located on both sides. Based on the laws of light refraction, one can theoretically prove that all axial rays, or rays traveling close to the main optical axis, incident on a thin collective lens parallel to its axis, converge in focus. Experience confirms this theoretical evidence.
By letting a beam of axial rays parallel to the main optical axis onto a thin, double-angle lens, we find that these rays come out of it in a beam that diverges. In the event such a diverging beam enters our eye, it will seem to us that the rays come from 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 located on both sides of it. When a beam of rays that are parallel to any of the secondary optical axes is directed at the lens, this beam, after its refraction, converges on the corresponding axis at the point of intersection with the focal plane.
The optical power of the lens is such a quantity that is inverse to its focal length. We determine it using the formula:
1 / F = D.
The unit of measurement for this force is called diopter.
1 diopter is the optical power of a lens with a focal length of 1 m.
For convex lenses, this force is positive, and for concave lenses it is negative.
For example: What will the optical power of a spectacular convex lens equal to, if F = 50 cm - its focal length?
D = 1 / F; by condition: F = 0.5 m; from here: 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 of which the lens consists, and 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 substance, R1, 2 are the radii of curvature of the surface. The radii of convex surfaces are considered positive, and concave - negative.
The nature of the image of the object received from the lens, i.e., its size and position, depends on the location of the object relative to the lens. The location of the object and its value can be found using the lens formula:
1 / F = 1 / d + 1 / f.
To determine the linear magnification of the lens, we use the formula:
k = f / d.
The optical power of the lens is a concept that requires detailed study.