# Focal Length of a Spherical Mirror

**Formula:** `f = r / 2`

The focal length ** f** of a spherical mirror is half of its radius of curvature

**. This means that if you have a spherical mirror, which could be either concave or convex, the distance from the mirror's surface to its focal point is half the distance of its radius. For a concave mirror, this focal length is positive, indicating that the focal point is in front of the mirror. For a convex mirror, the focal length is negative, indicating that the focal point is behind the mirror, and the value for radius should be taken as negative. This formula is a simplified model and assumes that the mirror has a small aperture compared to its radius of curvature, which is often referred to as the paraxial approximation.**

*r*Tags: Physics, Optics, Spherical Mirror, Focal Length