Plane mirror


A plane mirror showing the virtual image of an urn nearby.
A diagram of an object in two plane mirrors that formed an angle bigger than 90 degrees, causing the object to have three reflections.

A plane mirror is a mirror with a flat (planar) reflective surface.[1][2] For light rays striking a plane mirror, the angle of reflection equals the angle of incidence.[3] The angle of the incidence is the angle between the incident ray and the surface normal (an imaginary line perpendicular to the surface). Therefore, the angle of reflection is the angle between the reflected ray and the normal and a collimated beam of light does not spread out after reflection from a plane mirror, except for diffraction effects.

A plane mirror makes an image of objects in front of the mirror; these images appear to be behind the plane in which than mirror lies. A straight line drawn from part of an object to the corresponding part of its image makes a right angle with, and is bisected by, the surface of the plane mirror. The image formed by a plane mirror can either be virtual (meaning that the light rays do not actually come from the image) or real (meaning that the light rays do actually come from the image). But it is always upright, and of the same shape and size as the object it is reflecting. A virtual image is a copy of an object formed at the location from which the light rays appear to come. Actually, the image formed in the mirror is a perverted image (Perversion), there is a misconception among people about having confused with perverted and laterally-inverted image. If a person is reflected in a plane mirror, the image of his right hand appears to be the left hand of the image.

Plane mirrors are the only type of mirror for which a object always produces an image that is virtual, erect and of the same size as the object in all cases however same is possible for other types of mirror but only for a specific condition. However the focal length of a plane mirror is infinity;[4] its optical power is zero.

Using the mirror formula:

1/u +1/v =1/f

1/u=-1/v since [1/f=0]

=> u=-v

Concave and Convex mirrors (spherical mirrors)[5] are also able to produce virtual images similar to a plane mirror. However, the images formed by them are not of the same size as the object like they are in a plane mirror. In a convex mirror, the virtual image formed is always diminished, whereas in a concave mirror when the object is placed between the focus and the pole, an enlarged virtual image is formed. Therefore, in applications where a virtual image of the same size is required, a plane mirror is preferred over spherical mirrors.


A ray diagram for a plane mirror. The incident light rays from the object create an apparent mirror image for the observer.

A plane mirror is made using some highly reflecting and polished surface such as a silver or aluminium surface in a process called silvering.[6] After silvering, a thin layer of red lead oxide is applied at the back of the mirror. The reflecting surface reflects most of the light striking it as long as the surface remains uncontaminated by tarnishing or oxidation. Most modern plane mirrors are designed with a thin piece of plate glass that protects and strengthens the mirror surface and helps prevent tarnishing. Historically, mirrors were simply flat pieces of polished copper, obsidian, brass, or a precious metal. Mirrors made from liquid also exist, as the elements gallium and mercury are both highly reflective in their liquid state.

Relation to curved mirrors

Mathematically, a plane mirror can be considered to be the limit of either a concave or a convex spherical curved mirror as the radius, and therefore the focal length becomes infinity.[4]

See also


  1. ^ Moulton, Glen E. (April 2013). CliffsNotes Praxis II: Middle School Science (0439). Houghton Mifflin Harcourt. ISBN 978-1118163979.
  2. ^ Saha, Swapan K. (2007). Diffraction-Limited Imaging with Large and Moderate Telescopes. World Scientific. ISBN 9789812708885.
  3. ^ Giordano, Nicholas (2012-01-01). College Physics. Cengage Learning. ISBN 978-1111570989.
  4. ^ a b Katz, Debora M. (2016-01-01). Physics for Scientists and Engineers: Foundations and Connections. Cengage Learning. ISBN 9781337026369.
  5. ^ "2.2 Spherical Mirrors - University Physics Volume 3 | OpenStax".
  6. ^ Kołakowski, Leszek (September 2000). Science and Technology Encyclopedia. University of Chicago Press. ISBN 9780226742670.