(i) Pole (P) is generally taken as the mid point of reflecting surface.

(ii) Centre of curvature (C) is the centre of the sphere of which the mirror is a part.

(iii) Radius of curvature is the radius of the sphere of which the mirror is a part. Distance between P and C.

(iv) Principal Axis is the straight line connecting pole P and centre of curvature C.

(v) Principal focus (F) is the point of intersection of all the reflected rays which strike the mirror (with small aperture) parallel to the principal axis. In concave mirror it is real and in the convex mirror it is virtual.

(vi) Focal length (f) is the distance from pole to focus.

(vii) Aperture is the diameter of the mirror.

(viii) If the incident ray is parallel to the principal axis, the reflected ray passes through the focus. (Fig (a))

(ix) If the incident ray passes through the focus, then the reflected ray is parallel to the principal axis (Fig.(b))

(x) Incident ray passing through centre of curvature will be reflected back through the centre of curvature. (Fig.(c))

Tracing for spherical mirror

(1) A ray going through centre of curvature is reflected back along the same direction.

(2) A ray parallel to principal axis is reflected through the focus, and vice-versa. Also, mutually parallel rays after reflection intersect on the focal plane.

(3) A ray going to the pole and the reflected ray from it make equal angles with the principal axis.

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