DOUBLE CONCAVE || Highly polished, double concave lens SPHERICAL || Lens measures 3" (75mm) in diameter 300MM FOCAL LENGTH || Diverging lens with a 300mm 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