To produce a parallel beam of light from a concave mirror, the distance at a which the lamp should be placed from the mirror is equal to
Answer Details
To produce a parallel beam of light from a concave mirror, the distance at which the lamp should be placed from the mirror is equal to the focal length.
This is because a concave mirror has a focal point, which is the point where all incoming parallel rays of light converge after reflection. By placing the lamp at a distance equal to the focal length, the rays of light will reflect off the mirror and converge at the focal point, resulting in a parallel beam of light.
Placing the lamp at any other distance would result in the rays of light either converging or diverging after reflection, which would not produce a parallel beam.
Therefore, option A, the focal length, is the correct answer.