Question

Three thin lenses are combined by placing them in contact with each other to get more magnification in an optical instrument. Each lens has a focal length of $3 \,cm$. If the least distance of distinct vision is taken as $25 \,cm$, the total magnification of the lens combination in normal adjustment is

• A. 9
• B. 26
• C. 300
• D. 3

Answer 1

Answer: (B)

Combined focal length $\left(\frac{1}{F}\right)=\frac{1}{F_{1}}+\frac{1}{F_{2}}+\frac{1}{F_{3}}$
$\Rightarrow \frac{1}{F}=\frac{1}{3}+\frac{1}{3}+\frac{1}{3}$
$\Rightarrow F=1 \,cm$
Magnification of the lens combination in normal adjustment is
$M=1+\frac{D}{F}=1+\frac{25}{1}=26$