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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.

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.

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 $$cm$$.