1 Chills and Tingling fingers and Skin symptoms (4 causes) 2  Chills and Tingling fingers and Temperature symptoms (4 causes) 3  Chills and Tingling fingers and Body symptoms (3 causes) 4  Chills and Tingling fingers and Leg symptoms (3 causes)  Chills and Tingling fingers and Limb symptoms (3 causes)
From the geometry of the spherical mirror, note that the focal length is half the radius of curvature: As in the case of lenses, the cartesian sign convention is used here, and that is the origin of the negative sign above. The radius r for a concave mirror is a negative quantity (going left from the surface), and this gives a positive focal length, implying convergence.
The focal length of a lens is the distance from the center of the lens to the point at which it focuses light rays. The shorter the focal length, the more powerful the lens. (It's easy to see why: an ordinary piece of glass would be like a lens of infinite focal length and wouldn't bring light rays to a focus at all.