Transparent bodies, in which at least one surface is curved, are called lenses. Most often there are lenses that are symmetrical about the optical axis. Optical features of the lens depend on the radius and type of curvature.

Collecting lens

In convex, or collecting, lenses, the middle is thicker than the edges. A parallel beam of light, such as a sunbeam, falls on a convex lens. The lens collects a beam of light at the focus of F. The distance from the middle plane to the focus is called the focal length of the lens f. The shorter it is, the greater the optical power of the lens. This force is measured in diopters.

Take a lens with a focal length of 0.5 meters. Then the optical power of the lens is equal to one divided by the focal length: 1 / 0.5 m = 2 diopters.

Collecting lenses with different focal lengths

Diffusing lens

Concave or diffuse lens edger are those lenses whose edge thickness is greater than the thickness in the middle.

In this case, a parallel beam of light will scatter. At the same time it will seem that the beam of light comes from a single point, which is called an imaginary focus. The focal length in this case will be negative and, accordingly, the optical power of the scattering lens will also be negative.

Take a lens with a focal length of -0.25 meters. Then the optical power will be equal to: 1 / -0.25 = -4 diopters.

Imaginary focus lens

  • The principle of building the image collecting lens
  • The collecting lens gives a valid image. Only it will be turned upside down.
  • Image formed by a collecting lens

If we want to get a more accurate image, then, knowing the length of the focus, we can build this image. For this we need three rays.



A beam propagating parallel to the optical axis, refracted in the lens and passing through the focus, is called a parallel beam.

Parallel beam

The beam passing through the center of the lens is called the main beam. He is not refracted.

Main beam

The beam that passes in front of the lens through the focus and then spreads parallel to the optical axis is called the focus beam. At the point where all three rays intersect, there will be the clearest image.

Focus beam

If the distance from the object to the lens is very large, then the distance from the image of this object to the lens will be much smaller, i.e. image will be reduced.

If the distance from the object is twice the focal length, then the image will be the same size as the object itself, and it will be at the double focal length behind the lens.

Image formation, the size of which corresponds to the object

If we bring the subject closer to the focus, we will get an enlarged image that is at a great distance on the other side of the lens.

Larger image

If the subject is right in focus or even closer to the lens, then we get a fuzzy image.

Written by
Kristel Nice