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Electron Microscopy.

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Introduction

Electron Microscopy. Electron microscopy is a method of imaging that uses an electron microscope to enlarge small specimens by a greater magnification and resolution than conventional light microscopes. The photographs produced of specimens viewed with an electron microscope are call electronmicrographs. Magnification is the increase in apparent size of the specimen and resolution (also called resolving power) is the ability of the microscope to distinguish and produce separate images of closely placed objects. These two primary properties of electron microscopes make them extremely useful in the analysis and study of specimens. The obvious difference between electron microscopes and light microscopes is the medium through which each operates. Light microscopes work by using photons to produce an image whereas electron microscopes use electrons to produce an image. The magnifying power of a light microscope is limited by the wavelength of visible light so electrons are used instead because they have a much smaller wavelength so can therefore resolve much smaller structures. The resolving power of a microscope depends on the wavelength of the electromagnetic radiation used. ...read more.

Middle

The transmission electron microscope (TEM) allows us to see as separate structures, particles which are as close together as 2 nanometres. TEM can achieve magnifications up to around 1000000x and a resolution of about 2 nanometres. The TEM produces sharp definition at low magnifications but it can be used at high as well as low magnifications if preferred. The electronmicrograph from a TEM is dependent on electrons that do and do not pass through the specimen being sampled for it to be produced. Light areas on an electronmicrograph are produced when electrons have been able to pass through the specimen. However, if any electrons are to pass through the specimen then it would have to be a very thin specimen. The cells being imaged are not thin enough so they have to be cut into very thin sections, about the thickness of the wall of a soap bubble. Very small specimens such as viruses and large molecules are an exception and do not have to be sectioned because they are already thin enough. There are two methods of cutting the tissue into thin sections depending on the resultant image required. ...read more.

Conclusion

The scanning electron microscope (SEM) creates a magnified image of the surface of a specimen. A SEM can achieve up to around 100000x magnification. It is called a scanning microscope because it scans the surface of the object bit by bit by using beams spanning to and fro across the surface of the complete object. Unlike the TEM, the SEM does not have such a high resolving power, however, there is no need for the specimens to be cut into thin sections prior to scanning and the SEM can also accommodate larger specimens than the TEM. The SEM works on the principle of scanning the electrons that bounce off the surface of the sample as well as any electrons that are emitted too. However, the atoms in organic molecules do not have very high atomic numbers and so do not scatter electrons. To adjust this, the surface of the specimen has to be coated with a thin film of gold. Electron-dense areas on the specimen scatter the electrons from the microscope and produce dark areas on the electronmicrograph. As you can see the advent of electron microscopy was a great feat. It allows theories based on the molecular scale to be proven and so many more findings and conclusions to be found. ...read more.

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