Electron microscopes

• Transmission

  Produces a beam of electrons that is focused onto the specimen by a condenser electromagnet. The beam passes through a thin section of the specimen (they absorb electrons s d appear dark). Other parts of the specimen allow the electrons to pass through and do they appear bright. The image is produced on a screen and this can be photographed to give a photomicrogram
• Scanning electron microscopes

  It directs a beam of electrons onto the surface of the experiment from above rather than penetrating it from below. The beam is passed back and forth across a portion of the specimen and the pattern is dependent on the contours of the specimen. This can build up a 3D image of the specimen by the computer analysis of the pattern of the scattered electrons and secondary elements produced.
• Limitations of the transmission electron microscope 


  • the whole system must be in a vacuum and therefore living organisms cannot be studied

  • a complex staining process is required even though images are not in colour

  • the specimen must be extremely thin

  • the image may contain artefact- they are things that result from the at the specimen is prepared. It is therefore not always easy to be sure what is actually in the specimen
• Limitations of the scanning electron microscope


  The limitations are the same, except for the fact that specimens do not need to be extremely thin as the electrons do not penetrate

  The basic SEM has a lower resolving power than the TEM but is still 10x better than a light microscope