The plano-concave lens converts a beam of parallel light into a diverging light, which is focused in reverse at the virtual focal point of the plano-concave lens with a negative focal length, where one side is flat and the other side is concave. A plano-concave lens emits a collimated beam from a virtual focus, usually used in a Galilean beam expander. When used, the concave surface faces the direction of the incident light, and the side of the plane faces the focal plane to be processed. Plano-concave lenses have negative focal length and negative spherical aberration, which can be used to offset aberrations of other lenses in the system.