Question

# The objective lens of a compound microscope produces magnification of 10. In order to get an overall magnification of 100 when image is formed at 25 cm from the eye, the focal length of the eye piece should be

A
4 cm
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B
10 cm
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C
259 cm
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D
9 cm
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Solution

## The correct option is C 259 cmFor final image to be formed at 25 cm, the image formed by the objective has to lie within the focal length of the eyepiece. In this case, Magnification m=mo(1+Dfe) where m0 is the magnification of the objective, D is the near point of the eye (25 cm) and fe is th focal length of the eyepiece. Given m=100, m0=10 Substituting in the equation, 100=10(1+25fe) From this fe=259 cm

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