radius of curvature in case of plane mirror
Plane mirror Mathematically, a plane mirror can be considered to be the limit of either a concave or a convex spherical curved mirror as the radius, and therefore the focal length, becomes infinite.[4]
Radius (bone) The radius or radial bone is one of the two large bones of the forearm, the other being the ulna. It extends from the lateral side of the elbow to the thumb side of the wrist and runs parallel to the ulna. The radius is shorter and smaller than the ulna. It is a long bone, prism-shaped and slightly curved longitudinally.