Question

If the distance of an object is 23 cm in the focal length is -7 what is the distance of the image

Answer 1

Given:

$d_o=23\:\rm cm$

$f=-7\:\rm cm$

The thin lens equation says

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

So

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

$$d_i=\frac{fd_o}{d_o-f}=\frac{-7*23}{23-(-7)}=-5.4\:\rm cm$$