• anonymous
We have to study electron microscopes in Biology and the two types we have too look at are: Scanning Electron Microscopes (SEM) and Transmission Electron Microscopes (TEM) the both have the same resolution (0.2nm) but differing magnifications (100,000x and 500,000x respectively) as the images are computer generated, surely the resolution is the only limiting factor, therefore they should have equal maximum attainable magnifications. My question is, why are they different? (thank you greatly)
  • Stacey Warren - Expert
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  • katieb
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  • anonymous
I'm a little but confused by your question but I believe the answer can be found in the functional differences between an SEM and a TEM. Both microscopes function by sending directed beams of electrons at the specimen and reading the scattering or disruption of electrons which hit the specimen. However, in the TEM, the electron beam either passes through the tissue sample or bounces back resulting in the black part of the image. In the SEM, the electrons are not actually being read as they pass through the sample, but rather as they are scattered after bouncing off the surface resulting in a surface only look at the sample. It is important to note here that the sample must be coated with a substance to both to allow scattering of electrons and to prevent interaction of the sample with the electron beam. Generally speaking, the resolution in TEMs is limited by the ability of the scope to confine the electron beam to a specific area. The greater the ability of the scope to limit the beam to a smaller beam, the greater the resolution. In SEMs, the resolution is limited both by the coating substance and by system limitations similar to those found in the TEM. TEMs can be used to image individual atoms mainly because they rely on the passage of electrons and the interaction of atoms and electrons is actually a major part of what is being imaged. However, in SEMs, the coating substance interferes with this since the electrons interact with the coating and with the sample. Additionally, since scatter and not interaction is being measured, and individual atoms are unlikely to cause enough scatter to draw an accurate picture, it makes sense that TEMs would have a higher inherent ability to magnify than SEMs.

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