- Particles have a wavelength which is dependant on their size
- Electrons have a wavelength which makes them useful for imaging purposes
Electron microscopy works due to something called the wave-particle duality theory. Now a full explanation of this theory is beyond both the limits of this subject and of my ability. However, the basics of this theory is that both waves and matter share some of the same properties. For example, both you and I sitting in front of our computers have a wavelength, although it is so large in comparison to our sizes that we would never detect it. In the case of very small particles such as an electron however, this wavelength becomes very useful. This is because when an electron is accelerated by a p.d. (potential difference or voltage) of 500,000 V, then it's wavelength is almost 2 pm (10-12m).
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There are unfortunately problems in this because bouncing electrons off the object is only half the issue. The other half is catching them afterwards. To this end magnetic fields are set up which focus the electrons in much the same way as a lens does light waves. It is this collection of the diffracted electrons which limits the magnification of the final image. These electrons can also be absorbed or scattered by the sample, they can also be effected by air and so the entire process must be carried out in a vacuum.
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The cathode an anode generate the potential difference necessary to accelerate the electrons. The first set of magnets then select those electrons with the correct energy before they arrive at the object to be viewed. The electrons which are diffracted by the object are then focused by a series of magnetic lenses which produced the magnified image on the screen at the bottom. |
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