The distance between an object and its real image in a convex lens is 40cm. If the magnification of the image is 3, calculate the focal length of the lens.

A. 6.5 cm
B. 7.5 cm
C. 8.5 cm
D. 4.5 cm

Correct Answer: Option B

Explanation

u + v = 40

v/u = 3

v = 3u

u + 3u = 40

4u = 40

u = 10cm

v = 3u = 30cm

f = uv/(u+v)=10(30)/(10+30)=300/40

= 7.5 cm