Transmission Electron Microscope (TEM)

Transmission electron microscopy is explained in some school biology courses e.g. AS Biology in the UK.

• What is a transmission electron microscope ?
  Answer: A large piece of scientific equipment (see the video below) that forms detailed images (micrographs) of extremely small objects or areas of objects by passing a beam of electrons through a very thin slice of the object or area of interest.
  
  
• What is transmission electron microscopy ?
  Answer: The use of a transmission electron microscope (TEM) to study minute structures by taking transmission electron micrographs of thin sections of materials and/or studying diffraction patterns produced using a TEM.
  
  
• What is a transmission electron micrograph ?
  Answer: An image generated by a transmission electron microscope.

It is useful for biologists to know about TEMs because they may need to look at micrographs, including transmission electron micrographs, to study biological structures. In order to interpret images accurately it helps, and is sometimes necessary, to understand how the image was formed.

Advantages of Transmission Electron Microscope

The advantages of a TEM over a light microscope are the advantages of electron microscopes (in general) over light microscopes - just brief key points appear below, see compare light vs electron microscopes for further details.

• Resolution: Electron microscopes can resolve very much greater detail than light microscopes because the electron beam has a much shorter wavelength than the (comparably longer) wavelength of visible light that forms the image in a light microscope.
• Magnification: Electron microscopes can form much higher magnification images (magnification = image size / object size) due to the high magnification 'power' of the electromagnetic lenses, especially the magnetic objective. The high magnification power is possible due to high voltage applied to the electromagnetic objective.

Limitations of Transmission Electron Microscopes (TEM)s

• It is not possible to observe living specimens because the whole system must be in a vacuum in order for the image to be formed.
• No colour images: Transmission Electron Micrographs (images generated by TEMs) are greyscale images* - not colour images, or even false-colour images. *Sometimes said to be "black and white" but "greyscale" is a more accurate description.
• "Flat" 2-D micrographs are generated by TEMs because the specimens must be extremely thin in order for electrons to penetrate them, hence there is no "thickness" to image in 3D, or more accurately, from which to form images that provide depth information - even if in the form of a 2-D image e.g. printed in the same way as digital photographs.
• Considerable preparation of specimens is involved. This includes staining specimens using specially selected chemicals. See histology stains. The specimens must also be very thin.
• Artefacts may appear in micrographs so accurate interpretation of TEMs may require considerable expertise and experience in addition to knowledge of the process used to prepare specimens and then form specific images. (Artefacts are features in micrographs that are present due to the preparation processes rather than due to the specimen itself.)

Video about Transmission Electron Microscopy

The short (5 minute) presentation below includes transmission electron microscope pictures and some micrographs formed by transmission electron microscopes. It also describes some of the basic physical principles that explain how and why transmission electron microscopes work.

Taking a few minutes to watch this TEM video clip may help you remember the key points about TEMs and appreciate their physical size and complexity - which shows why light microscopes are used in schools and colleges while electron microscopes are generally used in research environments.

Some of this information in the video exceeds the requirements of AS Biology. It is probably intended for physicists e.g. towards the end of the clip the narrator talks about the production of diffraction patterns being one of the "most important capabilities" of TEMs. Thanks to Diego Palacio, Joshua Justice and Prof David Lederman of West Virginia University (USA) for making this clip available on YouTube.

See also more information about How does a Transmission Electron Microscope work ? and Compare light microscopes with electron microscopes.