Q.72. Which one of the following will increase the resolution of a light microscope?
(A) Decreasing the numerical aperture of the objective lens
(B) Using an objective lens with a longer working distance
(C) Using a medium of higher refractive index
(D) Increasing the wavelength of light

The resolution of a light microscope improves with factors that minimize diffraction limits, as defined by Abbe’s equation:

d = 0.61 λ / NA, where d is the minimum resolvable distance, λ is the wavelength, and NA (numerical aperture) is n sin(θ) with n as the refractive index of the medium and θ as the half-angle of light collected.

Correct Answer

Option (C): Using a medium of higher refractive index boosts resolution by increasing the numerical aperture (NA), allowing finer details to be distinguished since higher n in NA = n sin(θ) reduces d.

Option Explanations

(A) Decreasing the numerical aperture: Lowers NA directly, enlarging the Airy disk and worsening resolution, as higher NA gathers more light obliquely for sharper images.

(B) Using an objective lens with a longer working distance: Prioritizes space over light collection; longer distance often means lower NA, reducing resolution despite easier sample access.

(C) Using a medium of higher refractive index: Enhances NA (e.g., oil immersion with n ≈ 1.52 vs. air’s 1.0), matching the objective for optimal resolution, as in oil-immersion lenses.

(D) Increasing the wavelength of light: Increases λ, raising d and blurring details; shorter wavelengths (e.g., blue light) improve resolution.

Resolution Factors Table