The radius of curvature of a plane mirror

The radius of curvature of a mirror is defined as the radius of the sphere from which the mirror is conceptually cut [t1, t5]. For spherical mirrors, the radius of curvature (R) is twice the focal length (f), expressed as R = 2f [c1, c2]. A plane mirror can be mathematically modeled as a spherical mirror with an infinitely large radius [t7]. As the surface of a mirror becomes flatter, the sphere it belongs to becomes larger; thus, a perfectly flat plane mirror corresponds to a sphere of infinite radius [t4, t7]. Consequently, the focal length of a plane mirror is also infinite, as it does not converge or diverge parallel light rays to a finite point [t4]. This geometric interpretation ensures that the laws of reflection remain consistent across all mirror types, where a plane surface represents the limit of zero curvature [c3, c4].

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > Activity 9.2 > p. 137
2. [2] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > What you have learnt > p. 159
3. [3] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 10: Light: Mirrors and Lenses > A step further > p. 160
4. [4] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > 9.1 REFLECTION OF LIGHT > p. 135