Question

ASSERTION
The mirrors used in search lights are parabolic and not concave spherical.
REASON
In a concave spherical mirror the image formed is always virtual.
Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
Assertion is correct but Reason is incorrect
Assertion is incorrect but Reason is correct

Answer 1

Search lights are designed to emit a powerful wide parallel beam of light. Concave mirrors suffer from spherical aberration. A source placed at its focus will not form a perfect parallel beam. Parabolic mirrors do not suffer from spherical aberration.

Complete step by step answer:
Concave spherical mirrors have spherical aberration. In other words, only the paraxial rays (the rays lying close to the principal axis throughout and making a small angle with it) coming from the source placed at the focus will contribute to a parallel beam. The marginal rays (the rays lying away from the principal axis and making a large angle with it) will not give a parallel beam. On the other hand, a parabolic mirror does not suffer from spherical aberration. It can give a wide intense parallel beam if a bright source is placed at the focus. Hence, searchlights use parabolic mirrors. The assertion is correct.
Concave mirrors can produce both real and virtual images. If an object is placed at a distance larger than the focal length, then a real inverted image is formed. The reason is incorrect.

Additional Information:
Searchlights were widely used in the First World War and the Second World War for night time attacks or for defense against bomber raids. They are now popularly used in fairs, festivals, movie premieres, concerts and advertisements. The searchlight at the top of the Luxor Hotel in Las Vegas is the most powerful in the world.

Note:
Parabolic mirrors suffer from chromatic aberration (coma). This causes a wedge shaped image for the objects that do not lie in the centre of the field. A telescope using parabolic reflectors makes the stars appear distorted having a tail similar to that of a comet.