Concave spherical mirror
= Concave spherical mirror
{title2=$f=R/2$}
A <concave spherical mirror> reflects at the inside of a spherical <surface> of <radius of curvature> $R$. For incident <light rays> near its <optical axis>, the <law of reflection> gives paraxial <focal length> $f=R/2$. Rays farther from the <optical axis> have different axial crossing positions, causing <spherical aberration>; one must specify a criterion to define the best full-aperture focus.