The Electron microscope

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The electron microscope

  • light microscopes have a poor resolution as a result of the relatively long wavelength of light.
  • in the 1930s the electron microscope was developed.
  • There are two main advantages of it: - has a high resolving power because of the short wavelength, - as the electron are negative charged the the beam can be focused using electron magnets.
  • The best electron microscopes can resolve things 0.1nm apart.
  • This is 2000 times better than a light microscope.
  • Because electrons are absorbed by molecules in the air a near vacuum has to be created within the chamber of an electron microscope in order for it to work effectively.
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Transmission Electron Microscope

  • Consists of an electron gun that is focused onto the specimen by an electron magnet.
  • In a TEM s beam passes through a thin section of specimen.
  • Parts of the specimen absorb electron and therefore appear dark.
  • Other parts allow electron to pass through and so appear bright.
  • The image can be produced on a screen and this can be photographed to give a photomicrograph.
  • The resolving power is 0.1 nm.
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Limitations of a TEM

  • The whole system must be in a vacuum and therefore living specimens cannot be observed.
  • A complex "staining" process is required and even then the image is in black and white.
  • The specimen must be extremely thin.
  • The image may contain artefacts. These are things that result from the way the specimen is prepared.

The image is a flat 2D image, but a 3D image can be built  by looking at a series of photomicrographs produced. But this is a slow and complicated process.

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The Scanning Electron Microscope

  • All the limitations of the TEM also apply to the SEM, except that the specimens don't need to be extremely thin as electrons do not penetrate.
  • The SEM directs a beam of electrons onto the surface of the specimen from above, rather than penetrating it from below. The beam is then passed back and forth across a portion of the specimen in a regular pattern.
  • The electrons are scattered by the specimen and the pattern of this scattering depends on the contours of the specimen surface.
  • A 3D image can be built by computer analysis of scattered electron and secondary electrons produced.
  • The basic SEM has a lower resolving power than the TEM, around 20nm.
  • But the resolving power is still 10 times better than a light microscope.
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