Electron microscopes.

They are two major types of electron microscopes the first type originally developed: The Transmission Electron Microscope, which is quite similar to the light electron microscope just that in this case electron beams are used instead of light beams. With this type of electron microscope the beam of electrons is passed through the specimen before being viewed. Only the electrons transmitted (That is the electrons that go through the specimen) are seen. This microscope allows us to see as separate structures, particles which are as close together as 2 nanometres, thus the resolution of this microscope is about 2 nanometres. Because of its high-resolution power the electron microscope can be used at low as well as high magnification.

The other type of electron microscope is the scanning electron microscope. In this type the electron beams is used to scan the surfaces of the structures and only the beams reflected by the surface is viewed. The scanning electron microscope gives a three-dimensional effect showing surface detail. Though both of these electronic microscopes are very effective, the scanning electron microscope can take longer specimens that the transmission electron microscope can. On the other hand the scanning electron microscope does not have such a high resolving power as the transmission electron microscope does.

Photographs of specimen viewed by electron microscope called electronmicrographs, Before any electronmicrographs can be viewed the specimen which is to be viewed has be prepared. These specimens need to be cut appropriately and stained. This specimen must be stained in order to improve contrast. The specimen must be very thin, meaning cells must be cut into very thin section about the thickness of the film around a soap bubble. On the other hand very small specimen such as viruses and large molecules don’t have to be sectioned. Their surfaces are coated with a thin film of gold. There are various ways in which specimen can be prepared. Mostly for transmission electron microscope, the specimen that is to be viewed, if it is an embedded tissue is cut using a knife made of glass called a microtome. The tissue is soaked in alcohol to dehydrate it. The tissue is then preserved using substances, which prevent enzyme action i.e. embedded in resin to make it hard this is to avoid it releasing any gases or vapours, which might hinder the clarity of the specimen. This might introduce artefacts.

In the technique known as the freeze fracture technique, the is dehydrated using substances like ethanol, the specimen is then frozen using liquid oxygen. This technique allows surface detail to be seen with the transmission electron microscope.  Also this technique prevents the fact that scanning electron microscope show only surface detail to be a hindrance. This is because the surface is split up and the split surface is coated using metal ions. An image is then formed from the electrons emitted or reflected from this surface. Metal ions are mostly used to coat specimen because they serve as a source of electrons.

As electron beams cannot pass through glass, and electron beams are not visible, the electron microscope use electromagnets to focus the image onto a fluorescent screen, which is equivalent to glass lens bending light. The areas on the specimen hit by electrons shine brightly giving overall a black and white picture. Light areas are produced when electrons are able to pass through the specimen. Electron dense areas scatter electrons and produce dark images on the electronmicrograph. Also a high intensity electron beam can destroy parts of the specimen.

Even though the electron microscope is very effective and has very high magnification and resolution power, it also has some problems. For example the specimen, the electron beam and the fluorescent screen must all be in a vacuum. This is because if the electron beams being used collide with air molecules they will scatter making it impossible to achieve a sharp picture. Also because water boils at room temperature in a vacuum the entire specimen must be dehydrated before being placed in the microscope. This means only dead specimen can be viewed, which is a great fall back to the electron microscope. Lastly unlike the light microscope the images produced are not coloured. They are either in shades of black or white. As at now many researchers are being carried out to be able to see living cells under these powerful microscopes.