First slide

Ray optics

Question

In a compound microscope, the focal lengths of two lenses are 1.5 cm and 6.25 cm. An object is placed at 2 cm from objective and the final image is formed at 25 cm from the eye lens. The distance between the two lenses is (in cm)

Moderate

A

6
B

7.75
C

9.25
D

11

Solution

Length of tube L= v₀ + u_e
$= \frac{u_{0} f_{0}}{u_{0} - f_{0}} + \frac{f_{e} D}{f_{e} + D}; \frac{1}{f_{0}} = \frac{1}{v_{0}} - \frac{1}{u_{0}} \frac{1}{f_{e}} = \frac{1}{- D} - \frac{1}{(+ u_{e})} = \frac{2 x 1.5}{2 - 1.5} + \frac{6.25 x 25}{6.25 + 25} = 11 cm$

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