5. The most important property of any microscope is its power of resolution, which is numerically equivalent to D,
the minimum distance between two distinguishable objects. D depends on three parameters namely,
the angular aperture (θ), the refractive index (µ), and wavelength (λ) of the incident light.
Below are given few options to increase the resolution of the microscope:
A. Decrease the value of λ or increase either µ or θ to improve resolution
B. Moving the objective lens closer to the specimen will decrease D and improve resolution
C. Using a medium with high refractive index between specimen and the objective lens to improve the resolution
D. Increase the wavelength of the incident light to improve resolution
Which of the following combination of above statements are correct?
A. A and C
B. B and C
C. A and D
D. C and D
The resolving power of a microscope improves by decreasing wavelength (λ) or increasing refractive index (μ) or angular aperture (θ), as per Abbe’s formula d = λ/(2μsinθ).
Correct Answer
A. A and C. Both statements correctly describe methods to decrease minimum resolvable distance D.
Option Breakdown
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A. Decrease λ or increase μ or θ: Correct. Formula d = λ/(2μsinθ) shows resolution improves (smaller d) when λ decreases or μsinθ (numerical aperture) increases.
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B. Moving objective closer decreases D: Incorrect. While closer positioning increases θ (angular aperture), this is already covered in A. B oversimplifies mechanical adjustment.
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C. High refractive index medium: Correct. Oil immersion (μ=1.51) vs air (μ=1.0) dramatically improves resolution (NA 1.4 vs 0.95).
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D. Increase wavelength: Incorrect. Longer λ worsens resolution (larger d); blue light (450 nm) resolves better than red (650 nm).
Microscope resolution parameters directly control minimum resolvable distance D = λ/(2μsinθ), fundamental for cell imaging in life sciences research.
Abbe’s criterion states resolving power RP = 2μsinθ/λ. Oil immersion objectives (μ=1.515, θ=72°) achieve ~0.18 μm vs dry objectives ~0.3 μm.
Resolution Improvement Methods
| Method |
Effect on Formula |
Resolution Change |
Example |
| A. ↓λ, ↑μ, ↑θ |
Directly improves RP |
Excellent (~2x) |
Blue light + oil |
| C. High μ medium |
↑ Numerical Aperture |
Excellent (1.5x) |
Cedarwood oil μ=1.51 |
| B. Move lens closer |
Minor ↑θ |
Marginal |
Mechanical only |
| D. ↑λ |
Worsens RP |
Poor |
Red light 650 nm |
Correct: A+C combination maximizes resolution for GATE exams.
Practical Applications
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Blue filter (450 nm) + 100x oil objective (NA=1.4): d ≈ 0.16<sup>μm</sup>
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UV microscopy: d ≈ 0.1<sup>μm</sup> (λ=350 nm)
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Electron microscope: d ≈ 0.5 nm (λ<sub>deBroglie)
Exam Tip: Memorize RP ∝ 1/λ, ∝ μ, ∝ sinθ. Oil immersion transforms resolution from routine to research-grade.
