If a convex lens of focal length 12cm is used to produce a real image four times the size of the object, how far from the lens must the object be placed?

A. 10 cm
B. 15 cm
C. 20 cm
D. 25 cm

Correct Answer:

Option B = 15 cm

Explanation

Magnifications = V/U = 4
∴ V = 4U
1/V + 1/U = 1/f
1/4u + 1/U = 1/12
5/4U = 1/12
4U = 12 x 5 = 60
∴ U = 15 cm