Image Formation in Plane Mirror

For a point object $A$ placed in front of a plane mirror, a ray $AO$ strikes the mirror at $O$ and reflects along $OB$, while a second ray $AD$ strikes the mirror normally at $D$ and reflects straight back along $DA$. Extending $BO$ and $DA$ backward, they meet behind the mirror at the virtual image point $A'$, from which the reflected rays appear to originate (no light physically passes through $A'$). Using the geometry of this construction -- specifically that $\angle AON=\angle DAO$ (alternate angles) and $\angle BON=\angle OA'D$ (corresponding angles) -- the triangles $\triangle ODA$ and $\triangle ODA'$ can be shown to be congruent, which forces $AD=A'D$. This proves algebraically what is visually intuitive: a plane mirror's …