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The Principles and Limitations of Scanning and Transmission Electron Microscopes

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Introduction

The Principles and Limitations of Scanning and Transmission Electron Microscopes

Electron microscopes were first developed due to the limitations of light microscopes (3). The smallest object that can be viewed by any microscope is half the wavelength of light used, and objects smaller than this cannot be seen. This is because the object has to be large enough to interfere with the waves radiation. Light has a wavelength between 400-700nm, so the smallest object that can be viewed using visible light is 200nm(3). By the early 1930’s all the possible scientific progress on understanding the inner parts of cells had been made, and scientists wanted to see more detail. Max Knoll and Ernst Ruska then developed the electron microscopes in 1931 (7).

Electron microscopes use the same principles as light microscopes, but a beam of electrons is used instead of a beam of light. Electron beams have a wavelength of about 0.005nm. This short wavelength means much smaller objects can be seen (3). The resolution of a microscope is its ability to distinguish between two objects that are very close together. Magnification shows the objects as one larger image. The shorter the wavelength, the better the resolution. Therefore the resolution of an electron microscope is better than a light microscope. The magnification is also better.

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Middle

The electrons are focused on to the specimen by a condenser. In electron microscopes this is an electromagnet which straightens and intensifies the beam of electrons (5). Electromagnets are used to focus the beam because they can deflect the negatively charged electrons (2). After passing through the specimen, the radiation is focused by an objective lens (5). The human eye cannot see electrons, so the projector lens focuses the final image on to a fluorescent screen which emits visible light where the electrons hit (4). This gives a black and white picture. It is possible to obtain a photograph of the final image by allowing the electrons to pass on to a photographic film. This produces an electron micrograph (3).

The main advantage of a TEM is the high resolution and magnification that can be obtained. This allows very detailed images of the cell structure to be seen. However one major disadvantage of TEMs is that electrons are easily scattered or absorbed by air molecules. Therefore, the specimens must be viewed in a vacuum. This, and the fact that the specimens must be cut very thinly means there are limitations on what can be viewed using an electron microscope. Usually only dead material can be viewed (2).

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Conclusion

e sublimes away to leave an

etched surface. A layer of carbon is then deposited on the surface to form a replica, and this

is coated in a layer of heavy metals. The heavy metals are used because they are good

emitters of secondary electrons. The specimen can then be destroyed and the replica

viewed (1).

SEMs show the surface of the structure and have great depth of field which enables them to

give a very detailed three dimensional image(3). They can be used to study relatively large

objects. The images are likely to look more lifelike because the replica is formed before any

chemical damage is done to the specimen (5). It is possible to view some organisms alive-

some insects can withstand the vacuum.(3). However, SEMs do not have the resolving

power of TEMs (5). External disturbances such as a stray magnetic field and mechanical

vibrations can cause image distortion, jagged edge lines and other phenomena (7).

References

1. Adds J.        (1992)  Tools, techniques and Assessment in Biology         Nelson

   Larkcom, E.

   Miller, R.

   Sutton, R.

2. Arms, K. (1988) Biology, a journey through life                                Saunders

   Camp, P.S.

3. Green, N. P. O (1997) Biological Science 1 (Third Edition)                Cambridge

    Soper, R.

    Stout, G. W.

    Taylore, D.J.

4. Jones, G. (1997) Advanced biology                                        Cambridge

    Jones, M.

5.Marcus Barbor (2000) Biology (2000 Edition)                                Collins

   Mike Boyle                                                                Advanced Science

   Mike Cassidy

   Kathryn Senior

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