An object 4 cm high is placed 15 cm from a concave mirror of focal length 5 cm. The size of the image is?

A. 2 cm
B. 3 cm
C. 4 cm
D. 5 cm

Correct Answer:

Option A =  2 cm

Explanation

(1)/v + (1)/u = (1)/f
(1)/v + (1)/15 = (1)/5
(1)/v = (1)/5 – (1)/15 = (3 – 1)/15 = (2)/15
∴ v = (15)/2 = 7.5 cm
magnification = (v)/u = (image size)/object size
(7.5)/15 = (X)/4
∴ image size. X = (7.5 x 4)/15 = 2 cm