Why are mirrors used in search lights parabolic and not concave sphric

1.	Why are mirrors used in search lights parabolic and not concave sphrical ?
Answer» 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 rendered 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.

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

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