Concave Mirror - Glass, dia 50mm, Focal length 75mm-EIS-PH0520B offers a thoughtfully designed, durable resource for hands-on learning. Optical Concave Optical Mirror 50mm diameter 75mm Focal Length This round concave glass mirror is designed for physics and educational uses. Designed for physics and educational uses. Because of their educational use design, there may be minor imperfections that do not affect physical function. Optically worked, silvered back with protective coating. Dia 50mm, focal length 75mm.