Understanding the Mirror Formula
The mirror formula is a vital concept in optics, specifically for spherical mirrors. It is expressed as:
\[ \frac{1}{f} = \frac{1}{u} + \frac{1}{v} \]
where:
- \( f \) = focal length
- \( u \) = object distance
- \( v \) = image distance
This formula is applicable to both concave and convex mirrors.

Application to Plane Mirrors
- Plane mirrors can be thought of as spherical mirrors with an infinite radius of curvature.
- When the radius of curvature approaches infinity, the focal length, which is half the radius of curvature, also approaches infinity.

Reasoning Behind the Application
- The characteristics of plane mirrors align with those of spherical mirrors:
- Plane mirrors reflect light in a manner that generates virtual images.
- The image formed is upright and of the same size as the object.
- Since the focal length of a plane mirror is infinite, the mirror formula still holds:
\[ \frac{1}{\infty} = \frac{1}{u} + \frac{1}{v} \]
This simplifies to:
\[ 0 = \frac{1}{u} + \frac{1}{v} \]
- This implies that the image formed by a plane mirror is located at the same distance behind the mirror as the object is in front of it.

Conclusion
- The mirror formula can be applied to plane mirrors as they can be considered as spherical mirrors with an infinite focal length.
- This understanding helps in analyzing the behavior of light and images formed by different types of mirrors effectively.