7. During microscopy, if the diameter of an object is decreased then its resolving power will be:
A. Increase in proportion to increase in radius
B. Decrease in proportion to increase in radius
C. Increase in an inverse manner with increase in radius
D. Decrease in an inverse manner with increase in radius
The resolving power of a microscope is an intrinsic property of the optical system, independent of the object’s size.
Correct Answer
None of the options are correct, but if forced to choose based on typical MCQ patterns, the question likely contains a conceptual error. Resolving power does not change with object diameter.
Detailed Explanation
Resolving power (RP) = 1/d where d = minimum resolvable distance = 0.61λ/NA. This depends on λ (wavelength), μ (refractive index), θ (aperture angle) – NOT object size.
Object diameter decreasing doesn’t affect microscope optics.
Option Breakdown
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A. Increase in proportion to radius: Wrong. RP is fixed; no proportional relationship exists.
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B. Decrease in proportion to radius: Wrong. No direct relationship; RP remains constant.
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C. Increase inverse with radius: Wrong. Smaller objects don’t improve microscope resolving power.
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D. Decrease inverse with radius: Wrong. RP independent of specimen size.
Microscope resolving power object diameter confusion arises in competitive exams, but resolving power is solely an optical property.
Abbe’s Limit: d = 1.22λ/(2NA) where NA = n.sinθ. A 10 μm bacterium or 0.1 μm virus faces same resolution limit (~0.2 μm light microscope).
True Resolution Factors
| Factor | Effect on RP | Example |
|---|---|---|
| Wavelength (↓λ) | ↑ RP | Blue (450 nm) > Red (650 nm) |
| NA (↑) | ↑ RP | Oil (1.4) > Air (0.95) |
| Object diameter | No effect | 1 μm or 10 μm = same dmin |
Conceptual Error: Question assumes object size modifies optics. Small objects challenge resolution but don’t change microscope capability.
Exam Trap Alert
GATE/CSIR tests this distinction:
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Magnification: Makes objects appear bigger
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Resolution: Reveals fine detail (fixed by optics)
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Object size: Only affects visibility within resolution limit
Answer Strategy: Recognize resolving power = microscope property, not specimen-dependent. Reject all size-based options.
