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Question: The magnifying power of a microscope with an objective of 5 mm focal length is 400. The length of it...

The magnifying power of a microscope with an objective of 5 mm focal length is 400. The length of its tube is 20 cm. Then the focal length of the eye-piece is

A

200 cm

B

160 cm

C

2.5 cm

D

0.1 cm

Answer

2.5 cm

Explanation

Solution

If nothing is said then it is considered that final image is formed at infinite and

m∞=(L∞−fo−fe)⋅Dfofe≃LDf0fem _ { \infty } = \frac { \left( L _ { \infty } - f _ { o } - f _ { e } \right) \cdot D } { f _ { o } f _ { e } } \simeq \frac { L D } { f _ { 0 } f _ { e } }

⇒400=20×250.5×fe⇒fe=2.5 cm\Rightarrow 400 = \frac { 20 \times 25 } { 0.5 \times f _ { e } } \Rightarrow f _ { e } = 2.5 \mathrm {~cm}