What is a lens?

It is an object usually made of glass and used to refract the rays falling on it. Lenses refract rays passing through them, bringing them closer or further away.

The lens is generally divided into:

1. Divergent Lens: Its edges are thicker than the middle.

2. Converging Lens: Its edges are thin from the middle.

Lenses are usually obtained by intersecting two spheres. The line joining the centers of the spheres that make up the lens is the principal axis of the lens. Rays falling parallel to the principal axis of the lens are refracted and continue on their way. The rays that continue their way after the refraction converge at a point on the principal axis. This point is the "focal point" of the lens. It is represented by the letter F.

The distance from the focal point to the lens is called the "focal length". The widest area of use of lenses is eyeglasses, binoculars, vision-related (optical) instruments such as microscopes and photographs, cinema cameras, etc. such tools. Depending on the location, one or more lenses can be used together. Most photographic lenses have several lenses together, which is called the "lens system".

 

The best example of lenses is the eye crystal in the structure of our eyes. The eyeball is a convex lens. All eyeglass lenses also constitute a lens. Lenses can be used alone or several lenses together form an optical instrument. Magnifying glasses, lenses and eyeglasses are examples of lenses used alone. Binoculars, microscopes, telescopes, cinema machines, cameras, and lens systems are optical arrangements.

The lenses give the desired favorable view of the object under examination. This image can be larger or smaller than the object, real or apparent, depending on the desired situation. The line joining the vertices of the surfaces limiting a lens forms the principal axis (optical axis) of the lens.

Lenses are divided into two groups, thin-sided and thick-sided:

Concave Lens

The middle is the lens, which is thicker than the edges. These lenses are called convergent lenses because they refract any ray coming into the lens and bring it closer to the optical axis. They refract the rays coming parallel to the principal axis and collect them at one point. This point is the focal point of the lens. There is a second focal point at the same distance and on the opposite side.

If the focal length f of a lens made of transparent material with a refractive index of n according to its environment and radii of curvature of its spherical surfaces is r 1 and r 2: the relation exists. If the distance of the object to the lens is U, the distance of the image to the lens is U', there is also a relation: U, U', f. The radius are taken as positive for convex (convex = convex) surfaces and negative for concave (pit = concave) surfaces.

If any surface is plane, its radius is taken as infinity. Focal length is included in the calculations as positive for convex lenses and negative for concave lenses. The distance of the real ones from the object and the image is taken as positive, and the distance of the apparent (visible) ones is taken as negative. The images of objects far from the focal point in convex lenses are always real and inverted.  The images of objects between the focus and the lens are always flat, apparent and larger than the object.

Concave Lens

Lenses that are thinner in the middle than the sides are called lenses. Concave lenses are called divergent lenses because they deflect any light coming into the lens from the optical axis. The image produced by concave lenses is always flat, apparent and smaller than the object. The inverse of the focal length of a lens in meters is called the convergence or power of that lens. This power is the power of breaking. The smaller the focal length of the lens, the greater its power or convergence.

Lens Defects (Aberration): Differences of the real image from the predictions of a simple theory are called defects or aberrations. The defects caused by the change of the refractive index of the lens depending on the colors of the light are called chromatic aberrations. Monochromatic aberration exists even when the light is monochromatic.

Colorless (chromatic aberration): Since the focal length of the lens depends on the refractive index, the seven-color rays of white light falling on the lens focus on points that move further and further away from the lens, from purple to red. Such images present difficulties for examination in telescopes, microscopes and similar systems. This defect is corrected with achromatic lenses. Achromatic lenses are lenses with one concave and one concave concave. The radii of curvature of the adjacent faces of the lenses are equal, and the other side of the convex lens is flat. One is made of crown, the other of flint glass.

Sphericity defect (Spherical aberration): The edges of the zoom lenses have a greater zoom power than their middle parts. For this reason, the rays falling on the lens, refracted at the edges, focus closer to the lens than those refracted in the middle. In this case, the on-screen image cannot be sharpened. This defect can be corrected in several ways.The most economical ones are to cover the thin parts of the lens with the diaphragm, or to use thin-sided and thick-sided lenses with different refractive indices. These lenses are made of flint and crown glass.

Astigmatism: If a large object is found in front of a lens, the sharpness of the image edges will deteriorate, as its far points from the optical axis will send oblique rays to the lens surface. This defect cannot be corrected in a single lens, but is corrected in lens systems, such as a camera or movie camera. In addition, lens defects such as coma, field curvature and distortion are also present.

Lens types

Microscope objectives: This type of lens is used to magnify small objects. Focal lengths usually range from 2 to 4.8mm. The magnification capacity is from 1000 to 5000. Telescope lenses: Telescope lenses are used to examine distant objects. These lenses have large diameters and focal lengths.

Spherical, coma, chromatic defects can be corrected in telescope lenses. However, astigmatic and field curvature usually cannot be corrected. Here, because the lenses are large, 50 cm or more chromatic defects appear, which is also impossible to correct.

Periscope: It is the most well-known type, which is generally used in submarines. However, it is also used in other sectors.

Camera lenses: It is generally used in cameras and television cameras. They serve to drop images of objects onto a film.

Glasses: These are the types of lenses used to correct eye defects.

Illumination lenses: Generally used to collect and focus light. This type of lens is used in projectors and illumination of microscopes.