Ray Optics

JEE Main level, numerical answer, medium

278 of 295

Question 278 of 295

An object viewed from a near point distance of 25cm25cm, using a microscopic lens with magnification 6,6, gives an unresolved image. A resolved image is observed at infinite distance with a total magnification double the earlier using an eyepiece along with the given lens and a tube of length 0.6m0.6m, if the focal length of the eyepiece is equal to ____ cm.cm.

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Answer: 25

For simple microscope,
m=1+Df0m=1+\dfrac{D}{{f}_{0}}
6=1+Df06=1+\dfrac{D}{{f}_{0}}
5=25f05=\dfrac{25}{{f}_{0}}
f0=5cm{f}_{0}=5cm
For compound microscope,
m=ℓ⋅Df0⋅fem=\dfrac{\ell \cdot D}{{f}_{0}\cdot {f}_{e}}
12=60×255⋅fe12=\dfrac{60\times 25}{5\cdot {f}_{e}}
fe=25cm{f}_{e}=25cm

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