Question

Find the position of the image formed by a lens of power +5 dioptres when the object is 20m away from it

Answer 1

The thin lens equation says

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

Hence,

$$\frac{1}{20}+\frac{1}{d_i}=5$$

$$d_i=0.202\:\rm m$$