Biconcave lens can disperse the parallel incident beam outward, with a negative focal length, consisting of two concave surfaces of equal curvature, commonly used for beam expansion, collimation, projection and expansion of the focal length of the optical system. Biconcave lenses have a negative focal length and are suitable for dispersing converging beams, also known as dispersive lenses or negative spherical lenses. A biconcave lens can diverge a collimated beam to an imaginary focal point and is commonly used in Galilean beam expanders.