This set of six glass lenses can be used to demonstrate optical properties. Each lens is 50 mm diameter and comes in a labeled envelope. You get one each of the following glass lenses with focal lengths as follows: double convex (+150 mm), plano-convex (+300 mm), concave-convex (+500 mm), double concave (-150 mm), plano-concave (-500 mm), and convex-concave (-300 mm). Combine lenses to demonstrate the properties of more complex optics. New.