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The electron microscope was invented in 1931 by two Germans, Max Knoll and Ernst Ruska. In
general an electron microscope works by speeding up electrons in a vacuum until their wavelength is
only one hundred-thousandth of white light. Beams of the electrons are then focused on a cell
sample and absorbed or scattered by the cell's parts to form an image on an electron-sensitive
Electron microscopes are very powerful and there have been occasions where it has been possible
to view objects as small as the diameter of an atom. Most electron microscopes that are used to
study biological matter can see down to 10 angstroms, which is approximately the same as
magnifying something 1 million times! Despite their power, it is not possible to view living specimens
using and electron microscope because they can't survive under such a high vacuum. This means that
it's not possible to show the ever-changing movements that characterize a living cell1.
There are two types of electron microscope; scanning and transmission. Scanning microscopes
produce 3D images and transmission microscopes produce 2D images. Here are some images from a
The Scanning Electron Microscope (SEM) shows very detailed 3D images at much higher
magnifications than is possible with a light microscope. The images are black and white because they
were not created using light. The specimens used in SEM must be dried out in a special way so they
don't shrivel up, and they also have to be made to conduct electricity. This is done using a machine
called a sputter coater which coats the specimens in a very thin layer of gold. The diagram below
shows an electron microscope, and an explanation as to what each part does is written below.
The air is pumped out of the vacuum column and the electron gun at the top
emits a beam of high energy electrons. The beam travels down through
both the condensing and objective lens to focus the electrons to a very fine
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The scanning coils move the focused beam across the specimen, back
and forth, row by row2.
Secondary electrons are knocked loose from the surface of the
specimen, as each electron beam hits a spot on it, and a detector
counts these electron and sends the signals to an amplifier2.
The final image is built up from the number of electrons emitted from each spot on the sample2.…read more
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