Question

A person uses $+1.5D$ glasses to have normal vision from $25\text{cm}$ onwards. He uses a $20D$ lens as a simple microscope to see an object. The maximum magnifying power, if he uses the microscope together with his glasses and without the glasses, are respectively:

• A. $6,9$
• B. $4,6$
• C. $5,8$
• D. $6,3$

Answer 1

The correct option is A $6,9$

The focal length of the glass (lens) used $f=\frac{100}{1.5}\text{cm}$.
If $y$ is the distance of near point then by the lens formula
$\frac{1}{y}-\frac{1}{-25}=\frac{1.5}{100}$
$y=-40\text{cm}$
The focal length of lens of microscope
$f_e=\frac{1}{p}=\frac{100}{20}=5\text{cm}$
Magnifying power of microscope together with glass
$M=1+\frac{D}{f_e}=1+\frac{25}{5}=6$
In case of without glass, $D=40\text{cm}=y$

$M^{\prime }=1+\frac{D}{f_e}=1+\frac{40}{5}=9$