Question

What focal length should the reading spectacles have for a person whose near point is `50 cm` ?

Answer 1

The book to be read is at `u = -25 cm` and the image is needed at `v = -50 cm`. Therefore, focal length (f) of the required lens is given by
`(1)/(f)=(1)/(v)-(1)/(u)=(1)/(-50)+(1)/(25)=(1)/(50)`
=`50 cm` (convex lens).