The radius of curvature (R) is the radius of the sphere of which a mirror or lens is a part. A plane mirror is a flat surface, meaning it is not curved. Therefore, it can be thought of as a part of a sphere with an infinitely large radius.

Mathematically, the relationship is given by the mirror formula: $\frac{1}{f}=\frac{2}{R}$. For a plane mirror, the focal length (f) is infinity. Substituting this, we get $\frac{1}{\infty }=\frac{2}{R}$, which simplifies to $0=\frac{2}{R}$. This is only possible if R is also infinity.

Final Answer: plane mirror