Question

The focal lengths of the objective and the eye-piece of a compound microscope are 2.0 cm and 3.0 cm respectively. The distance between the objective and the eye-piece is 15.0 cm. The final image formed by the eye-piece is at infinity. The two lenses are thin. The distances in cm of the object and the image produced by the objective measured from the objective lens are respectively

• A. 2.4 and 12.0
• B. 2.4 and 15.0
• C. 2.3 and 12.0
• D. 2.3 and 3.0

Answer 1

Answer: (A)

2.4 and 12.0

Answer 2

Given that $f_{o} = 2cm$, $f_{e} = 3cm$, $L_{\infty} = 15cm$

By using $L_{\infty} = v_{o} + f_{e}$ ⇒ $15 = v_{o} + 3$ ⇒ $v_{o} = 12cm$.

Also $\frac{v_{o}}{u_{o}} = \frac{v_{o} - f_{o}}{f_{o}}$ ⇒ $\frac{12}{u_{o}} = \frac{12 - 2}{2}$ ⇒ $u_{o} = 2.4cm$.