Spherical mirror formula relating an object distance ‘u’, image distance V and focal length of mirror T may be applied to a plane mirror when

The spherical mirror formula is given by 1/v + 1/u = 1/f, where 'u' is the object distance, 'v' is the image distance, and 'f' is the focal length. For a spherical mirror, the focal length is half the radius of curvature (f = R/2) [1]. A plane mirror can be considered a special case of a spherical mirror where the radius of curvature (R) is infinitely large, as its surface is flat rather than curved. Consequently, as the radius of curvature tends toward infinity, the focal length (f) also goes to infinity [1]. In this limit, the term 1/f becomes zero, simplifying the mirror equation to 1/v + 1/u = 0, or v = -u [1]. This result correctly describes the properties of a plane mirror, where the image distance is equal in magnitude but opposite in sign to the object distance.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > Activity 9.2 > p. 137