FUN + LEARN 50mm Premium Optical Glass Double Convex Lens for DIY Experiments (Pack of 2) Material: Optic Glass, Not Glass. 50mm diameter optical glass lens. Focal Length : 20cm. This lens is perfect for doing optical experiments comparing focal lengths and convex vs concave lenses. WE RECOMMEND YOU TO BUY AUTHENTIC PRODUCTS FROM FUNPLUSLEARN SOLUTIONS ONLY. WE DO NOT GUARANTEE QUALITY OF THE PRODUCT IF PURCHASED FROM 3RD PARTY SELLER.