Question Detail
Question
The real image of an object placed 20 cm from a concave mirror is three times the object. Calculate the focal length of the mirror.
Correct Answer
Option A is the correct answer.
Detailed Explanation
Magnification m = -v/u = -3 (real image is inverted, so m is negative). u = -20 cm (sign convention). m = -v/u => -3 = -v/(-20) => -3 = v/20 => v = -60 cm. Using the mirror equation: 1/f = 1/u + 1/v = 1/(-20) + 1/(-60) = -3/60 - 1/60 = -4/60 = -1/15. f = -15 cm. Focal length is 15 cm (magnitude).
Hint
Use m = -v/u for a real image. Then use the mirror equation 1/f = 1/u + 1/v.