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 the farthest distance at which he should keep the lens from the page so that can read the book when viewing through the magnifying glass ?
b. What is the maximum and the minimum angular magnification (magnifying power ) possible using the above simple microscope ?

Explanation

a. `(1)/(v) - (1)/(u) = (1)/(f), v = -25 cm, f = 5 cm`
-`(1)/(25) - (1)/(u) = (1)/(5) , - (1)/(u) = (1)/(5) + (1)/(25) = (6)/(25) `
u = `(-25)/(6) cm = - 4.2 ` cm
To see the object at the farthest point, its image must be formed at infinity.
i.e., v = `- infty " " therefore - (1)/(u) = (1)/(5) , u = - 5 ` cm
i.e., the object is placed at 5 cm from the magnifying glass
b. maximum angular magnification = m = `(D)/(|u|) = (25)/(|(25)/(6)|) = 6 `
minimum magnification = `(D)/(|u|) = (25)/(|5|)= 5 `