Spherical mirrors are a part of a sphere. Consider a sphere that is hollow in nature. Consider slicing a portion of a sphere and silvering the surface of the portions on the inner or outer surface, as shown in the below diagram.
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If a mirror is obtained from a sphere silvered on the outer side,  it is called a concave mirror. If a mirror is obtained from a silvered sphere inside, it is called a convex mirror.
Terms related to spherical mirrors:
Length AB is the measure of aperture, \(C\) is the centre of curvature, \(P\) is the pole, and \(PC\) is the principal axis.
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Important terms of spherical mirror
Aperture: The portion available for reflection is called aperture; \(APB\) is the aperture. 
Pole: It is the geometric centre of the reflecting surface. It is denoted by \(P\). 
Centre of Curvature: It is the centre of the sphere of which the mirror forms a part. \(C\) is the centre of curvature. 
Principal Axis: It is a straight line passing through the centre of curvature and the pole. The line passing through \(P\) and \(C\) in the figure is the principal axis.  
Radius of Curvature (\(R\)): It is the radius of the sphere of which the mirror forms a part. \(PC\) is the radius of curvature. 
Principal Focus: Consider a parallel beam of light incident on a spherical mirror.  In the case of a concave mirror, the parallel beam after reflection converges at a point F which is called the principal focus.
In the case of a convex mirror, it appears to diverge from the focus (\(F\)). Thus, a concave mirror is called a converging mirror, and a convex mirror is called a diverging mirror. 
Focal Length: It is the distance between the pole and the principal focus. \(PF\) is the focal length. It is denoted by ‘\(f\)’. It is measured in \(m\) or \(cm\).