The focal point of a [[Mirror]] is the center of its [[Spherical Abberation]]. For a mirror with center $\vec C$, curvature radius $r$, and incoming [[Quasi-Parallel]] [[Electromagnetic Radiation|Light]] [[Ray|rays]] [[Parallel|parallel]] to $\hat \ell$, the focal [[Point|point]] is equal to half of the radius offset from the center. $\huge f = \frac{r}{2} $ $\huge \begin{align} \vec f = \vec C + \frac{r}{2} \hat \ell \end{align}$ Whe discussing the [[Distance]] from $\vec C$ to the [[Focal Point]], we call this the 'Focal Length' of the mirror. Note that the direction $\hat \ell$ is the direction from the center along the [[Bisector]] between the center and the mirror. ![[Focal Point .excalidraw.svg]] Note that this line is also called the [[Spherical Principle Axis|Principle Axis]]. %%[[Focal Point .excalidraw.md|🖋 Edit in Excalidraw]]%%