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JEE Main 2020 (7 Jan, Shift 1) · Optical Instruments & the Eye

Optical Instruments & the Eye question from JEE Main 2020 (7 Jan, Shift 1)

Q31JEE Main 2020 · 7 JAN · SHIFT 1Medium

Optical Instruments & the Eye

If we need a magnification of 375 from a compound microscope of tube length 150 mm and an objective of focal length 5 mm, the focal length of the eye-piece, should be close to:

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Answer: (a)

Final image at near point: $M = \dfrac{L}{f_o}\left(1 + \dfrac{D}{f_e}\right)$ , $D = 250$ mm.
$375 = \dfrac{150}{5}\left(1 + \dfrac{250}{f_e}\right) \Rightarrow 1 + \dfrac{250}{f_e} = 12.5$ .
$f_e = \dfrac{250}{11.5} \approx 21.7$ mm.
So the answer is (a) 22 mm.

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