Why are mirrors used in search lights parabolic and not concave sphrical ?

Explanation

In search lights, we need an intense parallel beam of light. If a source is placed at the focus of a concave spherical mirror, only paraxial rays are redered parallel. Due to large aperture of mirror, marginal rays give a divergent beam.
But in case of parabolic mirror, when source is at the focus, beam of light produced over the entire cross-section of the mirror is a parallel beam.