There are two categories of microscopes based on the principle on which magnification is achieved, Light microscopes and Electron microscopes (EM). In Light or optical microscopes, magnification is obtained by a system of optical lenses using light waves. Electron microscope (EM) uses beams of electrons to produce images. In overall design, EM is similar to light microscopes with some differences (Refer: Electron Microscope vs Light Microscope)
Electron microscope was designed by Knoll and Ruska of Germany in 1932. There are two types of electron microscopes: TEM (Transmission Electron Microscope) and SEM (Scanning Electron Microscope)
Scanning Electron Microscope (SEM)
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Transmission Electron Microscope (TEM)
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Used to produce excellent images of the surfaces of cells and small organisms. Excellent for studying surface morphology of the organisms, cells or any suitable material under study
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Used to study the ultra structure of the cell and its components. It can see objects as small as a protein molecule or even at nano level. Provides details about internal composition of cells or any suitable material under study
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Electron beam scans over the surface of the sample
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Electron beam pass through the sample
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Based on scattered electrons or produces images by detecting secondary electrons which are emitted from the surface due to excitation by the primary electron beam
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Based on transmitted electrons or produces images by detecting primary electrons transmitted from the sample
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Comparatively low resolution than TEM; Resolution: 2nm(Average), 0.2nm (Special)
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High Resolution; Resolution: 10 nm (Average), 0.5nm (Special)
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Depth of field: High
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Depth of field: Moderate
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Magnifying power: 100,000X
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Magnifying power: 5,000,000X
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Specimen contrast: by electron adsorption
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By electron scattering
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Produces three-dimensional black and white images
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Produces two-dimensional black and white images
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Preparation technique: easy
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Skilled, very thin sample is required
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Preparation thickness: variable
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Very thin
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Specimen mounting: Aluminium stubs
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Thin films on copper grids
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Field of view: Large
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Limited
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Similarities between SEM and TEM
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· Type of object : Non living
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· Source of radiation: Electron
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· Medium: High vacuum
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· Nature of lenses: one electrostsastic and a few electromagnetic lenses
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· Magnification Adjustment: Current in the projector lens coil
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· Focusing: Current in the objective lens coil
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Image credit:1) http://remf.dartmouth.edu/imagesindex.html 2) https://www.paldat.org/pub/Tilia_platyphyllos/109978
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