Focal length is a term used in optics to describe the distance between the optical center of a lens or the pole of a mirror and its focus. It is denoted by the letter f and is measured in meters (m). The focal length of a lens determines the amount of bending of light rays passing through it, which in turn affects the image formed by the lens.

For a thin lens in air, a positive focal length is the distance over which initially collimated (parallel) rays are brought to a focus, or alternatively, a negative focal length indicates how far in front of the lens a point source must be located to form a collimated beam. For a converging lens, such as a convex lens, the focal length is positive and is the distance at which a beam of collimated light will be focused to a single spot.

It is important to note that the focal length of a lens depends on two factors: the curvature of the lens surfaces and the refractive index of the material the lens is made of. For a thick lens or an imaging system consisting of several lenses or mirrors, there are several related concepts that are referred to as focal lengths, such as effective focal length (EFL) and front focal length (FFL) .

In summary, focal length is a fundamental concept in optics that describes the distance between the optical center of a lens or the pole of a mirror and its focus. It determines the amount of bending of light rays passing through the lens and affects the image formed by the lens.