 Focal length

If you pass parallel light.

For a biconvex spherical lens, it is given by:where:

Focal length is a key concept in optics that refers to the distance between the lens or mirror and the point where parallel rays of light converge to a single point, known as the focal point. It is typically measured in millimeters (mm) and is a crucial parameter for both lenses and optical instruments, such as cameras and microscopes.