Electron microscopes allow the viewer to see much smaller objects than the original light microscopes. The wavelength of electrons is thousands of times smaller than the wavelength of light

Electron Microscopy

Electron microscopes allow the viewer to see much smaller objects than the original light microscopes. The wavelength of electrons is thousands of times smaller than the wavelength of light, with a ratio of around 0.01nm : 500nm, electron wavelength : light wavelength. This overcomes some of the problems faced with light microscopes, such as the depth of focus that can be obtained, the obstruction of objects and the blurred images that often occur due to the wavelengths of light. Electron microscopes can therefore show more detail then light microscopes and can be used to study much smaller objects in finer detail.

As electrons cannot be seen as light can, the image is shown on a screen when the electrons are detected within the microscope. A stream of electrons is formed at a hot cathode, or ‘electron gun’ inside the microscope, which is then ‘fired’ at the object being viewed using positive electrical charges. Metal apertures and magnetic lenses are used to confine the stream into a thinner, more focused beam. The beam is then directed onto the sample by magnetic lenses. The electrons are detected by a fluorescent screen or photographic plate at the front of the microscope, causing the ...

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Due to the finite energy of the electrons, care must be taken in the preparation of the specimen for viewing, and the subjects must be fixed to preserve their detail. The specimen must initially be fixed with a cross-linking agent and then buffered at the correct PH so no deformation occurs. The subject is then dehydrated and then covered in resin for a few days. This allows the resin to penetrate the whole of the specimen and embed the sample. Finally, the specimen is polymerised by heating, then the ‘cured’ block is ready for sectioning.

The specimens must be sliced very thinly to allow for the finite electron energy, so the ‘cured’ block is cut into ultrathin sections, which are then stained to help discern fine detail. They are stained with heavy metal to diffract the electron beam and produce contrast so the detail is easier to pick out when the image appears on the screen.

There are two types of electron microscope, the Transmission Electron Microscope and the Scanning Electron Microscope. In Transmission microscopes, the electrons travel through the subject and onto a detective surface. This allows an image to be compiled according to the detection of the transmitted electrons coming through the specimen. The subject therefore has to be extremely thin to allow the passage of electrons through it.

In Scanning microscopes, the electrons bounce off the object back onto a detective surface. An image is compiled on a screen according to, amongst other information, the distance the electrons have travelled. This allows for a more 3-Dimensional view of the subject than the images produced by the Transmission microscopes. It also means that the specimen can be much thicker than in Transmission microscopes as the electrons do not need to pass through the subject.

Peer Reviews

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pictureperfect

1st of Jul 2012

Quality of writing

There are some typing errors which could have been overcome if the candidate had throughly proof read their work. However in general there are few issues with spelling and grammar and the candidate describes the electron microscope accurately using appropriate technical terms.

Level of analysis

The candidate describes the electron microscope in detail and discusses its advantages in comparison to an optic microscope. As well as this they provide information on the different types of electron microscopes and how specimens must be prepared in order to be viewed. Therefore the candidate clearly shows a good level of understanding of this topic. As the information provided is quite descriptive, it would be appropriate to provide a labelled diagram of an electron microscope, the candidate could then refer to different sections of the diagram, this would help the reader understand how the microscope works.

Response to question

The candidate fails to include a suitable introduction to this piece of writing instead they simply dive straight into the essay and its main points. Having a good introduction is essential, it catches the readers attention and motivates them to read all of your essay rather than skim through it. Your introduction should include the topics you plan to discuss, it should be brief and to the point, save the detail for the bulk of the essay. On a positive note the candidate gives an in depth response to the set topic.

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