Question

When viewed using a convex lens, a 10 cm tall candle appears to be 30 cm
tall as shown in the figure. The focal length of the lens is 5 cm. What is the
object distance?

Answer 1

Explanation:

M = I/O = v/u = 30/10

therefore, v = 3u

f = +5 cm

$\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$

$\frac{1}{5} = \frac{1}{3u} - \frac{1}{u}$

$\frac{1}{5} = \frac{1 - 3}{3u}$

3u = -2 x 5

u = -10/3

= -3.33 cm

Therefore object is placed 3.33 cm in front of the lens.