Question

An object 15 cm high is placed 30 cm to the right of a lens. The image formed by this lens is inverted and 5.0 cm high. Find the image distance and the focal length of the lens.

Answer 1

$$M=-\frac{5}{15}=-\frac{d_i}{d_o}=-\frac{d_i}{30}$$

$$d_i=10\ cm$$

$$\frac{1}{f}=\frac{1}{30}+\frac{1}{10}\\f=7.5\ cm$$