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
Verified Correct Answer: B.
Physical Principles & Numerical Solution
u + v = 40
\(\frac{v}{u}\) = 3
v = 3u
u + 3u = 40
4u = 40
u = 10cm
v = 3u = 30cm
f = \(\frac{uv}{u + v} = \frac{10(30)}{10 + 30} = \frac{300}{40}\)
= 7.5 cm
Source: Myschool Past Questions Archive (Record #55005)