Q.82 Match the microscopes in Group I with their working principles in Group II
| Group I | Group II |
|---|---|
| P. Phase contrast Q. Dark field R. Bright field S. Electron microscopy |
I. Light reaches the specimen only from the sides II. Uses fluorescent dyes III. Difference in the refractive index of cells from their surrounding medium IV. Difference in contrast between specimen and its surrounding medium V. Uses electrons instead of photons as energy source |
Correct Answer: (B) P-III, Q-I, R-IV, S-V
Phase contrast microscopy exploits refractive index differences for contrast in transparent specimens. Dark field illuminates specimens only from the sides, scattering light for visibility. Bright field relies on natural absorption/scattering contrast. Electron microscopy uses electrons for superior resolution.
Option Analysis
Option (A) P-V, Q-II, R-III, S-V
Incorrect. Phase contrast (P) does not use electrons (V; that’s electron microscopy). Dark field (Q) lacks fluorescent dyes (II). Matches S-V but fails overall.
Option (B) P-III, Q-I, R-IV, S-V
Correct. Phase contrast-refractive index (III), dark field-side illumination (I), bright field-contrast difference (IV), electron microscopy-electrons (V).
Option (C) P-III, Q-II, R-V, S-I
Incorrect. Dark field (Q) does not use fluorescent dyes (II; that’s fluorescence microscopy). Bright field (R) is not electron-based (V).
Option (D) P-II, Q-I, R-III, S-V
Incorrect. Phase contrast (P) does not use fluorescent dyes (II). Bright field (R) relies on contrast differences (IV), not refractive index (III).
Introduction to Microscopes Working Principles
Mastering microscopes working principles is crucial for GATE Life Sciences, especially matching phase contrast, dark field, bright field, and electron types to their principles. This question tests core microscopy concepts: refractive index differences, side illumination, contrast variation, and electron beams. Option (B) perfectly aligns all matches.
Phase Contrast: Refractive Index Differences
Phase contrast (P-III) converts phase shifts from refractive index variations between cells and medium into amplitude differences, creating contrast in live, unstained transparent specimens like microbes. Invented by Zernike (Nobel 1953), ideal for bacterial motility without killing cells.
Dark Field: Side Illumination Only
Dark field (Q-I) uses oblique side lighting via a condenser annulus, blocking direct axial light so only scattered rays from specimen edges enter the objective—specimen glows bright against black background. Perfect for visualizing spirochetes like Treponema or thin fibers.
Bright Field: Contrast Differences
Bright field (R-IV), the standard light microscope, depends on natural absorption, reflection, or refraction creating intensity differences between specimen and surroundings. Requires staining for colorless microbes; resolution limited to ~0.2 μm.
Electron Microscopy: Electrons as Energy Source
Electron microscopy (S-V) accelerates electrons (shorter wavelength than light) in vacuum through electromagnetic lenses, achieving 0.1 nm resolution for ultrastructure. TEM for internal sections; SEM for surfaces.
GATE Exam Relevance
Microscopes working principles frequently appear in GATE XL-Microbiology/Biochemistry. Key distinctions: phase contrast exploits RI without dyes; dark field needs no staining; fluorescence (not listed) uses dyes. Memorize via tables for PYQs.
