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A microscope objective has a numerical aperture of 1.25. Using light of wavelength 5000 Angstrom, what is the minimum separation between two points so that they can just be resolved as distinct?

1. 0.24 micrometre
2. 0.48 micrometre
3. 0.12 micrometre
4. 0.38 micrometre

Correct answer: 0.24 micrometre

Solution

The resolving limit of a microscope is d = 0.61*lambda/NA. Here lambda = 5000 Angstrom = 5e-7 m and NA = 1.25. So d = 0.61*5e-7/1.25 = 3.05e-7/1.25 = 2.44e-7 m approx 0.24 micrometre.

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