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Curved Mirror

Curved mirror is a part of a transparent hollow sphere whose one surface is polished (Fig. 2).
Fig. 2
Concave mirror converges the light rays and used as a shaving mirror. In search light, in cinema projector, in telescope, by ENT specialists, etc.
Convex mirror diverges the light rays and is used in road lamps, side mirror in vehicles, etc.


  • Pole (P): Midpoint of the mirror
  • Center of curvature (C): Center of the sphere of which the mirror is a part.
  • Radius of curvature (R): Distance between pole and center of curvature. (Rconcave = –ve, Rconvex = +ve, Rplane = ∞).
  • Principle axis: A line passing through P and C.
  • Focus (F): An image point on principle axis for an object at ∞.
  • Focal length (f): Distance between P and F.
  • Relation between f and R: 113128.png
    (fconcave = –ve, fconvex = +ve, fplane = ∞)
  • Power: The converging or diverging ability of mirror.
  • Aperture: Effective diameter of light reflecting area. Intensity of image ∝ Area ∝ (Aperture)2.
  • Focal plane: A plane passing from focus and perpendicular to principle axis.

Sign conventions

Fig. 3
  • All distances are measured from the pole.
  • Distances measured in the direction of incident rays are taken as positive while in the direction opposite of incident rays are taken negative.
  • Distances above the principle axis are taken positive and below the principle axis are taken negative.

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