What is a trasparent medium with the curved surface that is used for focusing light?

A transparent medium with a curved surface used for focusing light is called a lens. Lenses can be convex or concave, depending on their shape, and they bend light rays through refraction. Convex lenses converge light to a focal point, while concave lenses diverge light. They are commonly used in optical devices such as glasses, cameras, and microscopes.