Here, consider lens or mirror at the origin. Now, all the distances measured in the direction of incident rays are positive and distances measured in the direction opposite to the direction of incident rays are negative. All distances have to be measured from the lens or mirror. For example object distance measured from lens or mirror is measured in the direction opposite to direction of incident rays hence, in these case object distance u is negative. The paraxial parallel rays are focused on the right hand side of convex lens. See, this distance f is measured in the direction of incident rays . Hence, focal length of convex lens is positive. But, we can see that focal length of concave mirror is negative. It’s radius of curvature is also negative. The radius of curvature of first surface of convex lens is positive, but radius of curvature of it’s second surface is negative. You can see that radius of curvature and focal length of concave mirror are negative.