A plano-convex lens will focus parallel rays of light into a point which is the focal point of the lens. It is characterized by a positive focal length, where one side is flat and the other is convex. When used, the convex side is usually facing the incident light, because the light incident on both sides of the plano-convex lens is slightly different in refractive index, so the focal length of the two sides is also different, in order to minimize the spherical aberration, should try to make the beam of light from the curved surface of the incidence. Plano-convex lenses are often used for aiming and focusing monochromatic light sources, converging parallel light, or converting scattered point light sources into parallel light.