Question

A person whose least distance of distinct vision is 20 cm uses a converging lens of focal length 5 cm as a magnifying glass. What is the magnification when the lens is held close to eye and the image is formed at the least distance vision

Answer 1

With the magnifying lens, the image is formed at the least distance of distinct vision, or at 20 cm. With the focal length of 5 cm it corresponds to the distance between the object and the lens of

$$o=\frac{fi}{f-i}=\frac{5\cdot20}{5-20}=-6.67\text{ cm}$$

(negative sign means virtual image).

The magnification is, thus:

$$m=\frac{i}{o}=-\frac{20}{-6.67}=3.$$