Any entity that has the ability to deflect light rays through it is called a lens. Likewise, lenses are transparent objects (made mainly of glass) composed of two surfaces, one of which is curved and the other is flat.

The word lens derives from the Latin lens or lentis, which means “lentil”, it has been baptized with this name due to the similarity (in its shape) with the well-known legume. Likewise, this word is ambiguous in gender, since it is usually used in the feminine to refer to optical glass, while when used in the masculine it refers (in some countries) to glasses or reading glasses or sun protection.

Lenses began to appear in the early 13th century, when manufacturers created small glass disks that could be mounted on a frame, with the aim of increasing the size of things and protecting the human eye from intense light. It was since then that the first reading glasses or book glasses were created.

Depending on the shape that the lenses take, they can be converging or diverging. Converging lenses are characterized by being thicker in the center and narrower at the edges. They get their name because the light rays unite or converge at a certain point that is called the image focus. These in turn can be; biconvex, planoconvex or concavo-convex.

On the other hand, diverging lenses become thicker at the edges, while they get narrower as the center is reached. They get their name, because they separate or diverge all the light rays parallel to the main axis that crosses them and their image focus is on the left, while the convergent ones have it on their right side. Diverging lenses can be; biconcave, planoconcave or convex-concave.

Currently there are lenses that are called artificial, since they are built with non-homogeneous artificial materials, causing their behavior to exhibit lower refractive indices, that is, a divergent artificial lens can become biconvex. This type of lens becomes very useful in microwaves thanks to its properties.