If a mirror has a focal length of −20 cm. It is a:

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Convex mirror

Concave mirror

Plane mirror

Lens

answer is B.

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Detailed Solution

Concept: A concave mirror has a reflecting surface that curves inward and away from the light source. Concave mirrors are used to reflect light inward to a single focus point. Concave mirrors, in contrast to convex mirrors, produce a range of pictures depending on the object's proximity to the mirror.
The focal length of a flat mirror is unlimited since an image can be created at any distance. In a plane mirror, the image is created at a distance behind the mirror that is equal to the object's distance from the mirror. An image is created in front of the focal point by a concave mirror.
The radius of curvature of a mirror determines its focal length. The formula for the focal length-radius relationship is

f = \frac{1}{R}

. where R stands for the mirror's radius of curvature and f stands for the mirror's focal length. R is negative for a concave surface, which results in a negative focal length and an upright, virtual picture.
Hence, the correct answer is option (2) .

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