Question

2. What is the image description of a 3 cm tall pen light when placed at distance of 30.5 cm, from a converging lens having a focal length of 10.2 cm?

Answer 1

Given:

$d_o=30.5\:\rm cm$

$f=10.2\;\rm cm$

$h_o=3\:\rm cm$

The thin lens equation says

$$\frac{1}{d_o}+\frac{1}{d_i}=\frac{1}{f}$$

$$d_i=\frac{fd_o}{d_o-f}=\frac{30.5*10.2}{30.5-10.2}=15.3\:\rm cm$$

Image size

$$h_i=-h_o\frac{d_i}{d_o}=-3*\frac{15.3}{30.5}=-1.51\:\rm cm$$

Image: inverted, reduced, real.