**i. **Unlike spherical shape, every point on a parabola is equidistant from a straight line and a point.

**ii.** Consider given parabola having RS as directrix and F as the focus. Points A, B, C on it are equidistant from line RS and point F.

**iii.** Hence A’A = AF, B’B = BF, C’C = CF, and so on.

**iv.** If rays of equal optical path converge at a point, that point is the location of real image corresponding to that beam of rays.

**v. **From figure, the paths A”AA’, B”BB’. C”CC’, etc., are equal paths when mirror is neglected.

**vi.** If the parabola ABC is a mirror then by definition of parabola the respective optical paths,

A”AF = B”BF = C”CF

**vii. Thus,** F is the single point focus for entire beam of rays parallel to the axis and there is no spherical aberration.

**Hence,** parabolic mirrors are preferred over spherical one as there is no spherical aberration.