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A cell structure has a size of 0.61 micrometre. To resolve this structure using orange light of wavelength 600 nm, what is the minimum numerical aperture (N.A.) the microscope objective must have? (Use the resolution limit d = 0.61*lambda/N.A.)

1. 0.30
2. 0.60
3. 0.75
4. 1.00

Correct answer: 0.60

Solution

The minimum resolvable separation of a microscope objective is d = 0.61*lambda/N.A. Solving for the numerical aperture: N.A. = 0.61*lambda/d = 0.61*(600 nm)/(0.61 micrometre) = 0.61*600/610 = 0.60. So an objective with numerical aperture at least 0.60 is required.

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