Ans. POLE
The midpoint of the curved surface of the spherical mirror is called pole of the
mirror
CENTRE OF CURVATURE
A spherical mirror is a part of a sphere. The centre of this sphere is called
centre of curvature.
RADIUS OF CURVATURE
The radius of the sphere of which spherical mirror is a part is called radius of
curvature.
PRINCIPAL AXIS
The line joining the centre of curvature and pole of the spherical mirror is called principal axis.
PRINCIPAL FOCUS
In case of a concave mirror:
The point where the rays of light parallel to the principal axis after reflection converge is called principle focus.
Concave mirror is also called corverging mirror. Since rays of light actually pass through the principal focus, therefore, it is called a real focus.
In case of a convex mirror:
The point where the rays of light parallel to the principal axis after reflection appear to come from situated behind the mirror is called principal focus.
Convex mirror is also called diverging mirror. Since rays of light do not actually pass through the principal focus but appear to do so, therefore, it is called a virtual focus.
*
FOCAL LENGTH
The distance from the pole to the principal focus measured along the principal axis is called focal length.