Question

The focal lengths of the objective and eye-lens of a microscope are 1 cm and 5 cm respectively. If the magnifying power for the relaxed eye is 45, then the length of the tube is

• A. 30 cm
• B. 25 cm
• C. 15 cm
• D. 12 cm

Answer 1

Answer: (C)

15 cm

Answer 2

Given that $f_{o} = 1cm$, $f_{e} = 5cm$, $m_{\infty} = 45$

By using $m_{\infty} = \frac{(L_{\infty} - f_{o} - f_{e})}{f_{o}f_{e}}$ ⇒ $45 = \frac{(L_{\infty} - 1 - 5) \times 25}{1 \times 5}$

⇒ $L_{\infty} = 15cm$