What Is The Focal Length Of A Plane Mirror: Explained Simply

How many focal lengths does a plane mirror have? A plane mirror is a type of reflective surface that produces an image by reflecting light. Unlike curved mirrors, such as concave and convex mirrors, which have a specific focal point, a plane mirror does not have a distinct focal length. Instead, its focal length is considered infinite. This means that parallel rays of light that strike a plane mirror will appear to diverge as if they are coming from a point at an infinite distance behind the mirror. In essence, the concept of focal length, which is essential for curved mirrors, does not apply to plane mirrors.

The focal length of a plane mirror, as determined by the mirror formula, is a unique and intriguing characteristic. In a plane mirror, the focal length is designated as infinity, which carries a profound implication for the behavior of light rays. Essentially, when light rays interact with a plane mirror, they follow a path that appears to extend infinitely behind the mirror’s surface. This phenomenon can be understood by considering a plane mirror as a specific type of spherical mirror with an exceptionally large radius of curvature (R), virtually approaching infinity. Consequently, the focal length of a plane mirror is half of this infinite radius of curvature, mathematically expressed as R/2, which ultimately leads to its intriguing property of being infinite. This means that light rays, when reflected by a plane mirror, diverge as if they were coming from a point infinitely far behind the mirror’s surface. This concept elucidates why we see virtual images that appear to be at the same distance behind the mirror as the object is in front of it. This unique optical behavior in plane mirrors serves as a fundamental principle in various applications, such as reflection in everyday mirrors.