Plano-concave lenses

Plano-concave lenses convert a beam of parallel light into divergent light, which is reversely focused at the virtual focus of the plano-concave lens. The focal length is negative, one side is flat and the other side is concave. Plano-concave lenses can diverge a collimated beam from the virtual focus and are usually used in Galilean beam expanders. When in use, the concave side faces the direction of the incident light and the flat side faces the focal plane to be processed. Plano-concave lenses have negative focal lengths and negative spherical aberrations, which can be used to offset the aberrations of other lenses in the system.