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How a Transmission Electron Microscope Works
In light microscopy it is the very nature of the visible light used that
limits resolving power. Electron microscopes use radiation of much shorter
wavelengths, namely electrons. Early in the 20th Century electrons were
shown to have wave properties and that the wavelength was governed by
their velocity (the faster a beam of electrons, the shorter its wavelength).
This discovery, plus the development of the magnetic lens (which is somewhat
analogous to glass lenses used in light optical systems) were key advances
in the development of the electron microscope.
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Electron microscope magnetic lenses consist of a coil of wire
shrouded in iron, which is continuous, but for a small gap in the
lens bore.
Transmission electron microscopes are constructed by stacking at
least five of these lenses together to produce the electron optical
column, with an electron source on top.
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The interior of the column is kept at high vacuum, because air molecules
can impede the path of the electron beam. A specimen on a support grid
is inserted into one of the magnetic lenses (the objective lens) and is
imaged at high resolution. The image produced can be further magnified
and viewed on a fluorescent screen, or can be recorded on film.
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