A spherical mirror has a reflecting surface shaped as part of a sphere. A concave mirror curves inward toward the incoming light, while a convex mirror curves outward.

Principal axis and focal point

The line through the mirror's center of curvature and vertex is the principal axis.

For rays close to the axis, a concave spherical mirror reflects parallel incoming rays toward a focal point. A convex mirror makes reflected rays diverge as though they came from a focal point behind the mirror.

In the paraxial approximation,

$$f=\frac{R}{2},$$

where $R$ is the radius of curvature.

Mirror equation

Object distance $d_o$, image distance $d_i$ and focal length $f$ satisfy

$$\frac1f=\frac1{d_o}+\frac1{d_i}$$

with a consistent sign convention.

Magnification

The transverse magnification is

$$m=\frac{h_i}{h_o}=-\frac{d_i}{d_o}.$$

Its sign indicates image orientation and its magnitude gives the image-size ratio.

Spherical mirrors are an idealized ray-optics model; rays far from the axis can produce spherical aberration because they do not converge at exactly the same point.