Question

A student uses a converging lens to produce an enlarged virtual image of a scale he whishes to read accurately. The focal length is 10cm. What is the appropriate distance between the scale and the lens?

Answer 1

F= 10 cm

d=25 cm ( optimal distance between the eye and the lens)

$\frac{1}{f}-\frac{1}{d}=\frac{1}{F} \implies f=\frac{aF}{a+F}=\frac{10·25}{25+10}=7.1 cm$

Answer f=7.1cm