A man with normal near point (25 cm) reads a book with small print using a magnifying glass : a thin convex lens of focal length `5 cm`.
answer of the following question (a) What are the closest and the farthest distances at which he can read the book when viewing through the magnifying glass ?
(b) What is the maximum and the minimum angular magnifications (magnifying powers) possible using the above simple microscope ?

Explanation

Given, Image distance, `v=-25cm`,
Focal length, `f=5cm`
(a) Cloest distance, `u=?`
`"Since "(1)/(v)-(1)/(u)=(1)/(f)`
`therefore" "u=(vf)/(f-v)=((-25)xx5)/(5+25)`
`=(-125)/(30)=(-25)/(6)~~-4.16cm`
This is the closest distance at which the man should keep the lens to read the book.
For Farthest distance, `u_(1)=?`
Image distance, `v_(1)=oo`
`"So "(1)/(v_(1))-(1)/(u_(1))=(1)/(f)`
`rArr" "u_(1)=-f=5cm`
So, 5 cm is the farthest distance at which he should keep the lens to read the book.
(b) Maximum angular magnification
`m=(D)/(u)=(25)/(25//6)=6`
Minimum angular magnification
`m=(D)/(u_(1))=(25)/(5)=5.`