Focal Length  (here concave mirror)




For a concave mirror when incident lights are parallel to the principal axis, after reflection light on the concave mirror converses to a point in the principal axis which is called principal focus.

  

 

Focal Length (here convex mirror)



 

In case of convex mirror when parallel beam of light incident on convex mirror after reflection the reflected light appears to diverse from a point F on its principal axis, which is called principal focus of the concave mirror.

 

 

Focal plane



For any convex or concave mirror, when parallel light (these are not parallel to the principal axis) incident on convex or concave mirror, after their reflection, they passing through a plane (in case of concave mirror) or appears to diverse from a plane (in case of convex mirror) Such a plane is called focal plane.


The plane which is perpendicular to the principal axis and passes through the focus is called the focal plane.