Optical Instruments: Practice Problem & Solution

Solution Explained:

To solve this problem, we apply the core principles of Optical Instruments. Understanding the underlying formula is key to arriving at the correct answer below:

Magnifying power $M = \frac{f_o}{f_e} = 9$, which gives $f_o = 9f_e$. Distance between lenses in normal adjustment is $L = f_o + f_e = 20 cm$. Substituting $f_o$, we get $9f_e + f_e = 20$, so $10f_e = 20 \Rightarrow f_e = 2 cm$. Then $f_o = 18 cm$.