The focal length of a plane mirror is: A) Positive B) Negative C) Zero D) Infinity

By BYJU'S Exam Prep

Updated on: September 25th, 2023

The focal length of the plane mirror is infinity. A plane mirror is a flat surface that reflects light. The parallel light beams are therefore reflected back parallel to one another in the case of a flat mirror. The distance between the focus and the pole of the mirrors is known as the focal length. The power of the mirror is often measured by focal length. The point at which parallel light rays will collide with a mirror. Given that the mirror is a planar surface with negligible curvature, the focal length’s curvature is also infinite.

What is the Focal Length of the Plane Mirror?

Focal length is the term used to define the power of a lens or a mirror. Keep in mind that the focal length of the flat surface mirror is always infinity. That means the distance between the focus and poles can’t be defined in the case of plane mirrors.

Important Facts about the Focal length of the Plane Mirror:

The relevant points about the focal length of the plane mirror are mentioned below. It will be useful tips for competitive exams.

  • A plane mirror is used to reflect light.
  • The focal length is infinity in the plane mirror.
  • It means the light rays passing through the mirror never meet at any point.
  • A planar mirror is a spherical mirror of an infinite radius of curvature (R).
  • So focal length is half of the radius of curvature (R/2), and hence it is infinite.
  • Focal length is the measure of distance between the focus and pole of the mirror.

The standard equation of focal length is given below.

  • 1/f =(1/u + 1/v)
  • f = focal length
  • u= object distance
  • v= image distance


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