Q5.The microscope that converts slight difference in the refractive index and cell density into easily observed difference in the light intensity is known as-
(A) Phase contrast microscope
(B) Bright field microscope
(C) Dark field microscope
(D) Transmission electron microscope
Answer: (A) Phase contrast microscope
Phase contrast microscopes convert slight refractive index and cell density differences into visible light intensity variations, enhancing contrast in transparent specimens without staining.
Option Breakdown
Phase Contrast Microscope (A)
Light passing through denser cell regions slows (phase shift), invisible to eyes; phase plate converts these shifts to amplitude changes appearing as brightness differences.
Annular diaphragm and phase ring create interference patterns revealing internal structures like nuclei and organelles in live cells.
Bright Field Microscope (B)
Standard light microscopes rely on absorption/scattering for contrast; transparent cells appear faint against bright backgrounds.
Requires staining, killing cells; cannot detect refractive index variations.
Dark Field Microscope (C)
Oblique illumination scatters light from specimen edges against dark background, highlighting outlines but not internal density gradients.
Produces bright specimen on black field; ignores phase shifts from refractive differences.
Transmission Electron Microscope (D)
Electron beams reveal ultrastructure at nanometer resolution using heavy metal stains; requires vacuum, fixation, dehydration.
Observes dead, sectioned samples; unsuitable for live cell refractive index imaging.
The microscope that converts slight difference in the refractive index and cell density into easily observed difference in the light intensity is phase contrast microscopy.
This schematic shows phase contrast light paths: direct (red) and diffracted (pink) rays through annulus, specimen, and phase plate creating interference contrast.
Phase Contrast Principle
Hollow light cone from condenser annulus hits specimen; denser areas retard light by ~λ/4; objective phase ring advances background light, causing constructive/destructive interference as bright/dark regions.
Live, unstained cells display natural contrast for motility and organelle studies.
Comparison with Alternatives
Bright field needs stains; dark field shows edges only; TEM kills cells for electrons.
Phase contrast uniquely translates phase shifts (invisible) to amplitude (visible).
Biological Applications
Essential for observing mitosis, bacterial flagella, tissue cultures without artifacts from fixation.
Nobel Prize-winning technique (Zernike, 1953) remains standard in cell biology labs.
This confirms A as the precise match for refractive index visualization.
