Focal length is the distance between the center of the mirror to the principal foci. In order to determine the focal length of a spherical mirror we should know the radius of curvature of that mirror. The distance from the vertex to the center of curvature is called radius of curvature.

The focal length is half the radius of curvature.

**Formula :**

F = ( R / 2 ) for concave mirror F = - ( R / 2 ) for convex mirror

Examples :

For a convex mirror Input : R = 30 Output : F = 15 For a convex mirror Input : R = 25 Output : F = - 12.5

## Python

`# Python3 program to determine ` `# the focal length of a ` `# of a spherical mirrorr ` ` ` `# Determines focal length of ` `# a spherical concave mirror ` `def` `focal_length_concave(R): ` ` ` `return` `R ` `/` `2` ` ` `# Determines focal length of ` `# a spherical convex mirror ` `def` `focal_length_convex(R): ` ` ` `return` `-` `( R` `/` `2` `) ` ` ` `# Driver function ` `R ` `=` `30` `; ` `print` `(` `"Focal length of spherical concave mirror is :"` `, ` ` ` `focal_length_concave(R),` `" units"` `) ` `print` `(` `"Focal length of spherical convex mirror is : "` `, ` ` ` `focal_length_convex(R),` `" units"` `) ` |

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Output :

Focal length of spherical concave mirror is 15 units Focal length of spherical convex mirror is -15 units

Please refer complete article on Program to determine focal length of a spherical mirror for more details!