Mirror Equation

What is Mirror Equation?

It is an equation relating object distance and image distance with focal length is known as a mirror equation. It is also known as a mirror formula.

Mirror Equation

In a spherical mirror:

  • The distance between the object and the pole of the mirror is called the object distance(u).
  • The distance between the image and the pole of the mirror is called Image distance(v).
  • The distance between the Principal focus and pole of the mirror is called Focal Length(f).

In ray optics, The object distance, image distance, and Focal length are related as,



  • u is the Object distance
  • v is the Image distance
  • f is the Focal Length given by \(f=\frac{R}{2}\)
  • R is the radius of curvature of the spherical mirror

The above formula is valid under all situations for all types of spherical mirrors (Concave and Convex) and for all object positions.

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Sign Conventions

New Cartesian Sign Convention

New Cartesian Sign Convention is used to avoid confusion in understanding the ray directions. Refer to the diagram for clear visualization.

  • All distances are measured considering mirror as the origin
  • A convex mirror has a negative focal length, whereas the concave mirror has a positive focal length.
  • Virtual distances are negative(-) whereas Real distances are positive(+)

Mirror Equation for concave mirror and Mirror Equation for a convex mirror

Mirror Equation for Concave and Convex Mirrors

The mirror equation \(\frac{1}{v}+\frac{1}{u}=\frac{1}{f}\) holds good for concave mirrors as well as convex mirrors.

Example of Mirror Equation

The radius of curvature of a convex mirror used for rearview on a car is 4.00 m. If the location of the bus is 6 meters from this mirror, find the position of the image formed.



The radius of curvature (R)= +4.00 m

Object distance(u) = -6.00 m

Image distance(v) = ?

Formula used:

\(f=\frac{R}{2}\) \(\frac{1}{v}+\frac{1}{u}=\frac{1}{f}\)


To calculate the Focal length of the given mirror, substitute the value of Radius of Curvature (R) in the \(f=\frac{R}{2}\). We get-


Since, \(\frac{1}{v}+\frac{1}{u}=\frac{1}{f}\) we can re- arrange it as –


On substituting the values in the above equation we get-

\(\Rightarrow\frac{1}{v} =\frac{1}{+2.00}-\frac{1}{\left ( -6.00 \right )}\) \(=\frac{1}{2.00}+\frac{1}{6.00}\) \(\frac{6+2}{2\times 6}=\frac{8}{12}\) \(v=\frac{12}{8}\)

= 1.5 meter.

The image is 1.5 meters behind the mirror.

The mirror is a polished surface which reflects the incident light to form the image. Here reflected light will have a wavelength and many other physical properties almost the same as that of the incident light.

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