Microscope Resolution Limits Visibility

Steps:

• Resolution is the minimum distance at which two points appear distinct, typically d ≈ λ/(2NA) for light microscopes.
• Convert units: 0.2 μm = 200 nm, matching the given resolution limit.
• An object the size of the resolution limit cannot be resolved as a distinct structure.
• Thus, the bacterium at 200 nm diameter is not visible under 200 nm resolution.

Why B is correct:

• Per Abbe's diffraction limit, structures equal to or smaller than the resolution d cannot be distinguished from background or resolved.

Why the others are wrong:

• A: As distance between points decreases, resolution improves (smaller d), not decreases.
• C: With 600 nm wavelength, resolution is ~300 nm (NA ≈1), so <300 nm separation appears as one.
• D: Longer wavelengths increase d, worsening resolution.

Final answer: B