Lens, Glass 38mm Set/6 These are great for optical benches, and other experiments in optics and come with a variety of useful focal lengths. One each: double convex, planoconvex, concavo-convex, double concave, plano-concave and convexo-concave. 6 demonstration glass lenses 38 mm in diameter and focal length of 20, 30 and 40. Qty :