Question

The near point of a patient's eye is 50.0cm. What focal length must a corrective lens have to enable the eye to see clearly an object 25cm away? Neglect the eye-lens distance

Answer 1

Given:

$d_i=-50.0\:\rm cm$

$d_o=25.0\:\rm cm$

The thin lens equation says

$$\frac{1}{f}=\frac{1}{d_0}+\frac{1}{d_i}$$

So

$$\frac{1}{f}=\frac{1}{25.0}+\frac{1}{-50.0}=\frac{1}{50.0}$$

$$f=50\:\rm cm$$