Question

An object that is 15 cm tall is placed 18 cm from a concave mirror. The real image is formed 20 cm from the mirror. What is the focal length of the concave mirror? Use the formula: f = dodi/do +di

Answer 1

Given:

$h_o=15\:\rm cm$

$d_o=18\:\rm cm$

$d_i=20\:\rm cm$

The mirror equation says

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

or

$$f=\frac{d_od_i}{d_o+d_i}=\frac{18*20}{18+20}=9.5\:\rm cm$$