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Question

# In a compound microscope the focal length of an objective lens is 1.2 cm and the focal length of the eye piece is 3 cm. When the object is kept at 1.25 cm in front of the objective lens, the final image is formed at infinity. Magnifying power of the compound microscope should be:

A
400
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B
200
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C
150
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D
100
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Solution

## The correct option is B 200Given: fo=1.2 cm, fe=3 cm and uo=−1.25 cm When the final image is formed at infinity, the magnifying power is given as : m=vo|uo|(Dfe) Using lens formula: 1vo−1uo=1fo ⇒1vo−1−1.25=11.2 ⇒1vo=11.2−11.25 ∴vo=30 cm Now, m=vo|u0|(Dfe) =301.25×(253)=200 Hence, (B) is the correct answer.

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