19.
In scanning electron microscope (SEM) to form an image of the specimen,
- (A) An electron should pass through the specimen
- (B) Electrons are scattered from the surface of the specimen
- (C) A thin film of heavy metal is evaporated
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A. Electron should pass through specimen: Incorrect. This describes Transmission EM (TEM), not SEM. SEM is surface imaging only.
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B. Electrons are scattered from surface: Correct. Primary beam → secondary electrons (topography) + backscattered electrons (composition) → image formation.
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C. Thin film of heavy metal evaporated: Incorrect. Sample preparation step (sputter coating with Au/Pt), not image formation mechanism.
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D. Specimens are stained: Incorrect. Light microscopy/biology TEM technique; SEM uses conductive coating, not staining.(D) Specimens are stained
Scanning Electron Microscopy (SEM) forms images by detecting electrons scattered from the specimen surface, unlike TEM which uses transmitted electrons.
Correct Answer
B. Electrons are scattered from the surface of the specimen. SEM uses secondary and backscattered electrons emitted from surface interactions.
Option Breakdown
Scanning electron microscope image formation relies on surface electron scattering, providing 3D topography at nanometer resolution.
SEM Process: 1-30 kV electron beam rasters surface → secondary electrons (SE) escape ~5 nm depth → Everhart-Thornley detector → grayscale image.
SEM vs Other Techniques
Technique Electrons Image Basis Sample Prep Resolution SEM Scattered (SE/BSE) Surface Au/Pt coating 5-10 nm TEM Transmitted Internal structure Ultrathin sections 0.2 nm Light Photons Absorption/phase Stains 200 nm Heavy metal film Coating only N/A Prep step N/A SE yield peaks at surface edges (bright), BSE shows atomic number contrast.
Key GATE Distinction
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SEM: Surface scattered → 3D appearance
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TEM: Transmission → 2D internal
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Coating: Prevents charging, NOT image formation
Exam Memory: SEM = Surface Electrons. TEM = Through.
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