Scanning Electron Microscope (SEM)

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

• What is a scanning electron microscope ?
  Answer: A large piece of scientific equipment that forms detailed images (micrographs) of surfaces of extremely small objects / specimens by passing a beam of electrons repeatedly across the surface of the object and imaging the electrons that are scattered from the surface into an electron detector.
  
  
• What is scanning electron microscopy ?
  Answer: The use of a scanning electron microscope (SEM) to study minute structures by producing scanning electron micrographs of the surfaces of areas of materials e.g. biological specimens using a SEM.
  
  
• What is a scanning electron micrograph ?
  Answer: An image generated by a scanning electron microscope.

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

Advantages of a Scanning Electron Microscope

The advantages of a SEM 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 Scanning Electron Microscopes (SEM)s

Limitations that also apply to transmission electron microscopes:

• 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.
• Considerable preparation of specimens is involved. This includes staining specimens using specially selected chemicals. However, the specimens do not need to be as thin as for use in a TEM. This is because in a SEM electrons do not penetrate the specimen or need to pass through it in order to form an image.
• 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.)

Limitations of SEMs compared with TEMs:

• Generally the image resolution of scanning electron microscopes (approx 20 nm) is less than for transmission electron microscopes. It is, however, about 10x higher than for light microscopes.

See also a comparison between animal cells, plant cells and bacteria cells.