Parallel light rays that are not parallel to the optical axis will meet on the focal plane. In this example, a simple concave lens is shown. Position F marks the position of the focal point.

The focal length ( f ) is the distance from a lens or mirror to the focal point ( F ). Light rays (of a single frequency) travelling parallel to each other, but not parallel to the optical axis of a lens or mirror, will meet on the focal plane. This is shown in the figure on the right.

Light rays that are parallel to each other, and parallel to the optical axis, will meet at the focal point.