A man with normal near point ( 25cm) reads a book with small print using a magnifying glass, a thin convex lens of focal length 5cm. (a) What is the closest and farthest distance at which he can read the book when viewing through the magnifying glass?

Q: 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.
(a) What is the closest and farthest distance at which he can read the book when viewing through the magnifying glass?
(b) what is the maximum and minimum magnifying power possible using the above simple microscope?

Sol: As for normal eye far and near points are ∞ and 25 cm respectively,

so for magnifier v_max = ∞ and v_min = -25 cm.

However , for a lens as ,

$\large \frac{1}{v} – \frac{1}{u} = \frac{1}{f} $

ie. $\large u_{min} = \frac{5}{-(5/25)-1} $

=(-25)/6 = -4.17 cm and u will be maximum when v_max = ∞