(a) Centre of curvature: center of the hollow sphere of glass of which the mirror is a part.

(b) Radius of curvature: radius of the hollow sphere of glass of which the mirror is a part.

(c) Pole: The centre of a spherical mirror is called its pole.

(d) Principal axis: The straight line passing through the centre of curvature and pole of a spherical mirror is called its principal axis.

(e) Aperture: The portion of a mirror from which the reflection of light actually takes place is called the aperture of the mirror.