Answer to Question #141588 in Optics for Sharfa

Question

An old person cannot see objects closer than 200 cm from his eye. Determine the lens he should wear to view objects at 25 cm from his eye.
5 Leel Ashokarathne
If the rays converging at a point not less than 150 cm behind the eye lens can be focused by the eye, find the visual range with spectacles.

Answer 1

The objects 200 cm away must be "shifted" by the lens to the distance of 25 cm from the eye. Thus, apply thin lens equation:

$$f=\frac{oi}{o+i}=\frac{(-2)\cdot0.25}{-2+0.25}=0.286\text{ m}.\\\space\\ P=\frac{1}f=3.5D.$$

The person is far-sighted, converging lenses are needed.