Class 10 Science

Cartesian Sign

In the case of spherical mirror all signs are taken from Pole of the spherical mirror, which is often called origin or origin point. This sign convention is known as New Cartesian Sign Convention.

Sign Convention for Spherical Mirror:

Sign is taken as – (negative) from pole of a spherical mirror towards object along the principal axis. This means sign is always taken as – (negative) in front of a spherical mirror. For example; the distance of object is always taken as – (negative) in case of both types of spherical mirror, i.e. concave and convex mirrors.

cartesian sign convention spehrical mirror

Fig: Cartesian Sign Convention

Sign in the case of concave mirror:

Sign in the case of a convex mirror:

Mirror Formula:

Mirror formula shows the relation among distance of object, distance of image and focal length in case of spherical mirror. All distances are measured from pole of the mirror.

The distance of object is denoted by u
The distance of image is denoted by v
Focal length is denoted by f


By knowing any two, the third can be calculated using the mirror formula.


Magnification is the relative ratio of size of image formed by a spherical mirror to the size of object. Magnification is generally denoted by letter ‘m’.

`text(Magnification m)=text(Height of image h’)/text(Height of object h)`

Or, `m=(h_i)/(h_o)`

Relation among magnification, distance of object and distance of image:

`text(Magnification m)=text(Distance of image)/text(Distance of object)=-v/u`

Thus, `m=(h')/(h)=-v/u`

Where; m = magnification, h' = height of image, h = height of object, v = image distance and u = object distance.


Cartesian Sign ConventionConventions for showing object distance, image distance, focal length, etc. with suitably adding positive or negative signs before the measurement.
Mirror FormulaThe equation that shows relationship among object distance, image distance and focal lenght.
MagnificationRatio between image size and object size, or between image distance and object distance.