A level Biology exam revision resources written by A level Examiners





 

There are two types of electron microscope. Both use beams of high velocity electrons instead of light to generate an image of the specimen. These beams are focused onto the specimen with powerful electromagnets. Because air particles would reflect the beams of electrons the entire assembly is placed in a vacuum.

Transmission electron microscopes pass a beam of electrons through the specimen. The electrons that pass through the specimen are detected on a fluorescent screen, on which the image is displayed. Very thin sections of specimen are needed for transmission electron microscopy, as the electrons have to pass through the specimen for the image to be produced. The transmission electron microscope gives the greatest resolution of the two types of electron microscope.

Scanning electron microscopes pass a beam of electrons over the surface of the specimen in the form of a 'scanning' beam. Electrons are reflected off the surface of the specimen, as it has previously been treated to reflect the electrons. The reflected electrons are focused on a fluorescent screen, in order to make up an image. Larger, thicker structures can thus be seen under the scanning electron microscope, as the electrons do not have to pass through the sample in order to form the image.

Electron microscopes provide high magnification and more importantly high resolution. The drawback to them is that the specimen must be highly prepared to be able to withstand the vacuum required and totally dehydrated, as water molecules would deflect the electron beam and blur the image.

Figure 3 : Electron Microscope
Electron Microscope


Relatively cheap
Rather expensive

Magnifies objects up to 1500 x
Magnifies over 500 000 x

Living specimens can be used
Not possible to use living specimens



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