Microscopes: Scanning Electron Microscope Essay

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Magnification: Is the degree to which the image is larger than the object itself.
For example (X10 , X100 etc.)

Resolution: Is the degree to which it is possible to distinguish between two objects, the higher the resolution the greater the detail.
Electron microscopes have some advantages over optical microscopes:
The biggest advantage is that they have a higher resolution and therefore also have a higher magnification (Up to 2000000 times).
Light microscopes can only show a useful magnification only up to 1000-2000 times. This is a physical limit imposed by the wavelength of the light.
Electron microscopes therefore allow for the visualisation of structures that would normally not be visible by optical microscopy.
Depending on the type of electron microscope, it is possible to view the three dimensional external shape of an object (SEM , Scanning Electron Microscope).
In SEM’s, due to the nature of electrons , electron microscopes have a greater depth of field compared to light microscopes. The higher resolution may also give the human eye the subjective impression of a higher depth of field compared to light microscopes.

Electron microscopes have a plethora of disadvantages as well:
They are extremely expensive.
The sample preparation is often much more elaborate. It is often necessary to coat the specimen with a very thin layer of metal (such as gold). The metal is able to reflect the electrons.
The Sample must be completely dry , meaning it is impossible to observe living specimens.
It is not possible to observe moving specimens (they are dead).
It is not possible to observe colour. Electrons do not possess a colour. Thus the image is only black and white but can sometimes be artificially coloured to give a better visual impression.
They require more training and experience in identifying artefacts that may have been introduced during the sample process.
The energy of the electron beam is very high. The sample is therefore exposed to high radiation and therefore unable to live.
The space requirements are high. They may need a whole room.
The maintenance costs are high.

When actually viewing specimens, it is sometimes necessary to stain them so they can actually be seen.Some specimens can be viewed directly but most biological specimens are not coloured and it is therefore difficult to observe details.Some material distorts so you cannot cut into it.
Staining-Coloured stains are chemicals. They bind to chemicals on or in the specimen.
Sectioning-Some specimens are embedded in wax and cut without distorting the structure.
STAINS:
Acetic Orcein : stains a dark red.
Gentian Violet: stains bacterial cell walls violet
Methylene Blue: stains living cells - nucleus=dark blue –cytoplasm=light blue.
Iodine: Stains living plant cells (e.g. starch grains) dark blue.
Eosin: Stains cytoplasm
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