DOUBLE CONVEX || Highly polished, double convex lens SPHERICAL || Lens measures 1.5" (38mm) in diameter is approximately 3mm thick 250MM FOCAL LENGTH || Converging lens with a 250mm focal length OPTICALLY WORKED GLASS || Made of high quality, optically worked glass. Highly polished with ground edges GREAT FOR PHYSICS CLASSROOMS || This optically true lens is excellent for physics classroom demonstrations in optics and light refraction