Q.81 Which of the following combinations would improve the resolution of a microscope?
(i) Increasing the half aperture angle of the objective lens
(ii) Decreasing the wavelength of the illumination source
(iii) Decreasing the numerical aperture of the objective lens
(iv) Decreasing the refractive index of immersion medium
(A) (i) and (ii) (B) (ii) and (iii) (C) (ii) and (iv) (D) (i) and (iii)
Answer: (A) (i) and (ii)
Microscope resolution improves when the minimum resolvable distance decreases, as defined by
Abbe’s formula:
d = 0.61λ / NA
where λ is the wavelength of light and
NA = n sin α (numerical aperture),
with n as the refractive index of the medium and
α as the half aperture angle.
Option Analysis
(i) Increasing half aperture angle (α)
Increasing α increases sin α, thereby increasing
the numerical aperture (NA). A higher NA allows the objective lens to collect more diffracted
light rays, which improves resolution.
(ii) Decreasing wavelength (λ)
Since λ is in the numerator of Abbe’s formula, decreasing the wavelength
directly reduces d. Shorter wavelengths (e.g., blue light) resolve finer
details than longer wavelengths (e.g., red light).
(iii) Decreasing numerical aperture
Lowering NA increases d, which worsens resolution. Therefore, this option
does not improve resolving power.
(iv) Decreasing refractive index of immersion medium
Reducing n (for example, switching from oil with
n ≈ 1.515 to air with n = 1.0) lowers NA and degrades resolution.
Improve Microscope Resolution: Half Aperture Angle and Wavelength Guide
Improve microscope resolution by optimizing key factors such as half aperture angle and
illumination wavelength. This guide explains how specific parameter combinations enhance
optical microscopy performance, especially for biology exams like
CSIR NET Life Sciences.
Understanding the resolution equation
d = 0.61λ / NA is essential. Resolution improves when NA increases
(via larger α or higher n) and when
λ decreases.
Resolution Factors
Half aperture angle
A larger α increases sin α, improving NA and enabling
wider light collection by the objective lens.
Wavelength
Shorter wavelengths (such as blue or UV light) resolve smaller structures, a principle
widely used in fluorescence microscopy.
Numerical aperture pitfalls
Decreasing NA directly impairs the ability to distinguish closely spaced objects.
Immersion medium
Higher refractive index immersion media (oil > water > air) maximize NA.
Decreasing n limits microscope performance.
Summary Table
| Factor | Effect on Resolution | Example |
|---|---|---|
| Increase α (i) | Improves | Oil objectives: NA > 1.4 |
| Decrease λ (ii) | Improves | 400 nm vs 700 nm halves d |
| Decrease NA (iii) | Worsens | Air objectives: NA ≤ 0.95 |
| Decrease n (iv) | Worsens | Air (1.0) vs oil (1.515) |
Conclusion: Only options (i) and (ii)
improve microscope resolution, which corresponds to option (A).
