Microscopes. Using electrons instead of light means that the illumination has a much shorter wavelength than light.

Microscopes

Cells can be seen with a light microscope but many structures within a cell - organelles - can only be seen clearly with an electron microscope. That is partly because an electron microscope has a greater magnifying power.

However, increasing only magnification has its limits because at some point magnification reveals nothing more - the details only look bigger and vaguer.

Magnification is how much bigger a sample appears to be under the microscope than it is in real life.

Overall magnification = Objective lens x Eyepiece lens

Using electrons instead of light means that the illumination has a much shorter wavelength than light. This is good because minute detail can be detected. We say that an electron microscope has a bigger resolving power than an light microscope

Resolution is the ability to distinguish between two points on an image. The resolution of an image is limited by the wavelength of radiation used to view the sample. This is because when objects in the specimen are much smaller than the wavelength of the radiation being used, they do not interrupt the waves, and so are not detected. The wavelength of light is much larger than the wavelength of electrons, so the resolution of the light microscope is a lot lower. The actual resolution is often half the size of the wavelength of radiation used. Thus, for the light microscope the maximum resolution is about 200nm - if two objects in the specimen are closer than 200nm in real life, then they will only show up as one object on the image. Using a microscope with a more powerful magnification will not increase this resolution any further. It will increase the size of the image, but objects closer than 200nm will still only be seen as one point.

Light microscopes

When a light microscope is in use the light rays are focused on to a transparent specimen and are focused again by two more lenses – the objective lens and the eyepiece lens. These two lenses produce a magnified image.

So long as the specimen is thin enough to allow light through, there are no limitations on what can be looked at using a light microscope.

Electron microscope

An electron microscope gives a higher magnification and higher resolution. Cells and organelles can be seen much bigger and in much more detail with an electron ...

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Light microscopes

So long as the specimen is thin enough to allow light through, there are no limitations on what can be looked at using a light microscope.

Electron microscope

An electron microscope gives a higher magnification and higher resolution. Cells and organelles can be seen much bigger and in much more detail with an electron microscope than with a light microscope.

As the name implies the electron microscope uses beams of electons instead of beams of light. A high voltage (50 000 v) is passed through a tungsten filament and a stream of electrons is emitted. These electrons are focused by a series of electromagnets arranged down the inside of the microscope.

The inside of the microscope column is evacuated. A vacuum is needed because electrons would be scattered by air molecules and an image would not be produced. A vacuum means that living specimens cannot be viewed.

Electrons cannot penetrate materials as a well as light rays can, so specimens have to be much thinner than those used in a light microscope. This and the fact the specimen has to be placed in a vacuum, places great limitations on what can be viewed under an electron microscope.

Heavy metals are used to stain parts of specimens. This increases the contrast between areas and gives a better picture.

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. 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.
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 been previously coated in heavy metals. These reflected electron beams are focused on a fluorescent screen in order to make up an image. Larger, thicker structures can be seen under the scanning electron microscope as the electrons do not have to pass through the sample in order to form the image. However the resolution of the scanning electron microscope is lower than that of the transmission electron microscope.

Peer Reviews

Here's what a star student thought of this essay

crystalclearmagic

14th of Mar 2012

Quality of writing

I spotted only one spelling mistake - interestingly this still can show the lack of proof reading, especially when the piece of work (I presume) was done on a word processor with spell check. The grammar and punctuation on the other hand, were sound. The student uses the correct technical terms expected for this level of qualification however, I would stress the importance of making sure that volts has a capital V and not a lowercase v as the student has written. This, I would say, is a major error for an AS or A2 student as the units of voltage are taught from around year 7 / 8 and the carelessness of forgetting to use the correct symbol is almost unacceptable. Again, this shows the importance of proof reading - something which perhaps some students still forget to do.

Level of analysis

The student shows the appropriate level of knowledge and depth of knowledge for AS level however, depending on the syllabus that the student studies, at times perhaps the student lacks some specific detail to reasoning why the microscope works or how it works. The student does address this, but I feel that they could have done this to more depth to ensure the highest grade is attained. The table at the end is very useful as a summary, but I do feel that a summary in words is also needed in this piece of work. Not only will it make it clear the final conclusion that the student has reached but it will also emphasise the understanding of the student to the examiner. This does not have to be a huge paragraph but simply a few sentences will make a big difference. The piece of work is also clearly laid out which helps examiners as they are guided through the piece of work whilst marking it.

Response to question

Whilst the student shows an obvious understanding of the topic and question asked, unfortunately they do not directly address their question anywhere in their answer. It is implied that the examiner will reach the same conclusion as the student after reading the entire piece of work. This should not be the case and the answer to the question should be evident from the start. Or at least, the student should aim to make it clear that the examiner will be guided throughout the piece of work to an appropriate conclusion. This said, the range of knowledge that the student demonstrates in the work is impressive and they can clearly distinguish between the two microscopes. Lastly, a quick note on the formatting of the piece of work. The pictures did not show up; most likely due to incompatibility of software. The usage of diagrams or pictures is good to illustrate the points made, however they are useless if they a) lack a caption and b) there is no mention of the pictures in the work.

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