Download Electron Microscopes - University of Toronto Physics

The electron microscope has done for the very small what the telescope
did for the vastness of space - revealed previously unseen worlds.
The theory behind the electron microscope was proposed in 1924, and
the first practical one was built in Germany in 1932. The first electron
microscope in North America was built at the University of Toronto in
1938.
While a light microscope magnifies about 1600 times - enough to see
bacteria 1/5000 cm long - the Centre's research electron microscope can
magnify an incredible 500,000 times on the fluorescent viewing screen.
The normal working magnification ra~ges from 2,800 to 90,000 times­
powerful enough to see tiny viruses one millionth of a centimetre across.
But magnification isn't the only criterion in microscope performance.
Equally important is resolving power. This is the ability to distinguish two
points in an object as being separate from each other. Your eye, for ex­
ample, has a resolving power of one hundredth of a centimetre - that is,
it can only distinguish between points that are at least one hundredth of
a centimetre apart. If two points are closer than this, they will be seen as a
single point, no matter how great the magnification may be.
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In microscopy, resolving power is expressed in angstroms (A). One angstrom
is 1/10P,000,000 cm. The best light microscope has a resolving power of
2000 A (1/50,000 cm.), which is 500 times greater than the eye. ~ut the
best modern electron microscope has a resolving power of only 2 A ­
1,000 times better than the best light microscope, and 500,000 times greater
than the naked eye! Electron microscopes have this greater resolving power
because they use electrons, which have shorter wavelengths than visible
light.
Although no one has yet been able to produce a microscope with sufficient
magnifying and resolving power to see a single atom, the electron micro­
scope can photograph molecules, which are groups of atoms.
'JA~ CANADIANS IN PHYSICS University of Toronto
Graduate Students Make History
Although many research groups around the world
were attempting to design and build electron
microscopes in the 1930s, the first high-resolution
electron microscope that was practical and there­
fore became the prototype for the first commercial
instrument was designed, built, and tested by two
graduate students at the University of Toronto.
James Hillier and Albert Prebus are shown in the
photograph with the electron microscope that they
built in 1938. Hillier continued to perfect and use
the electron microscope while he completed his
Ph.D. degree. In 1940, Hillier joined the staff of the
Radio Corporation of America (RCA) in Camden,
New Jersey, where he continued to improve the
electron micro­
scope. In 1969,
H~lier became
the executive
vice president in
charge of research
and engineering
for RCA. In this
position, he was
responsible for
all of the research,
development,
and engineering
programs.
The race to build electron microscopes was
based on Davisson and Germer's verification of the
wave properties of electrons. Electron microscopes
have much greater resolving power than light
microscopes, due to their very short wavelengths.
Resolving power is the ability to distinguish two
or more objects as separate entities, rather than
as one large object. If the distance between two
objects is much less than the wavelength, a micro­
scope "sees" them as one particle, rather than as
two. You can magnify the image to any size, but all
that you will see is one large, blurred object. Since
the shortest wavelength of visible light is about
400 nm and electrons can have wavelengths of
0.005 nm, electron microscopes could theoretically
have a resolving power more than 10000 times
greater than light microscopes. In practice,
however, electron microscopes have resolving
powers about 1000 times greater than light
microscopes.
The diagram shows the typical design of a
transmission electron microscope. The barrel of
the microscope must be evacuated, because
electrons would be scattered by molecules in the
air. Electrons would not penetrate glass lenses, of
course, so focussing is accomplished by magnetic
fields created by electromagnets. These magnetic
"lenses" do not have to be moved or changed,
because their focal lengths can be changed simply
by adjusting the magnetic field strength of the
electromagnets. Since electrons cannot penetrate
glass, the extremely thin electron microscope
specimens are placed on a wire mesh so that the
electrons can penetrate the areas between the
tiny wires.
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The photograph at the beginning of this chapter
was produced by a scanning electron microscope.
These instruments function on a very different
principle than do transmission electron micro­
scopes. A very tiny beam of electrons sweeps back
and forth across the specimen, and electrons that
bounce back up from the sample are detected.
Scanning electron microscopes were first devel­
oped in 1942, but they were not commercially
available until 1965.
McGraw·Hill Ryerson Physics 12
TRANSMISSION ELECTRON MICROSCOPE ~\-f---
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There are two types of electron
microscopes. The earliest, and most
common, is the transmission elec­
tron microscope (TEM). Using
electromagnetic lenses, it focuses
a beam of electrons which is
transmitted through an extremely
thin specimen into another series
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enlarge the specimen image carried
by the beam and project it onto a
fluorescent screen where it can be
seen. Alternatively, the image can
be trained onto a photographic
plate or 35 mm film to obtain a
permanent record of the image.
Specimens examined in the TEM
must be very thin, so that the
electron beam can penetrate them_
To prepare biological samples for
this microscope, specimens are
embedded in hard plastic, then
sliced by a diamond or glass blade
into sections that are only a few
hundred angstroms thick. The
delicate slices are floated off the
knife edge onto water and are
picked up on a thin copper grid,
3 mm across. The specimen is
then stained with heavy metals
(such as uranium or lead) to
improve contrast among its var­
ious parts. The grid with its
stained specimen is inserted into
the TEM's vacuum chamber,
where it is struck by the beam.
The TEM works on the same principles as the light micro­
scope, except that it uses electrons instead of light to produce an image, and magnetic lenses instead of glass lenses to focus the beam. Tran smission
Electron Micro scope
SCANNING ELECTRON MICROSCOPE With the advent of the scanning electron microscope (SEM) the three dim­
ensional appearance of microscopic objects could finally be seen. The SEM,.
however, can only be used to observe surface features because electrons
that pass through the specimen are not seen. The SEM was developed in
1938 by M. vonArdenne, but the first commercial model was introduced
only in 1965.
The SEM works in a similar way to a television picture tube. The micro­
scope's condenser lenses focus the electron beam into a fine ray that scans
the surface of a specimen, (just as an electron beam moves back and forth
across the face of a television tube.) As electrons strike the specimen in
the microscope, they are scattered
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or knock secondary electrons from
the sample. The scattered and sec­
ondary electrons are picked up by
a detector and transmitted onto
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a cathode-ray viewing screen, like
a television set. Crevices in a
specimen produce fewer detect­
able electrons whereas projections
are highlighted. The result is an
image with three-dimensional
appearance.
Because the SEM doesn't need
thin sections of specimens, it
can deliver pictures of whole
organisms, from protozoa to in­
sects. Samples are covered with
a thin coating of precious
metals before being exam­
ined, to sharpen the image.
The resolution of the SEM
is much better than the
light microscope, but poor­
er than the TEM. Commer­
cial instruments usually
operate at 100 A(one
millionth of a centimetre.)
Vacuum
System
Opening of the fallopian tube
near the ovaries 5177X
Scanning Electron Microscope
USES OF ELECTRON
MICROSCOPES
In medicine, electron micro­
scopes are used to study cell­
ular changes in diseases such
as cancer, and to diagnose
blood and viral diseases, var­
ious types of muscular dys­
trophy and kidney disorders.
The TEM is used in dark field
microscopy to view the struc­
ture of large molecules such as
DNA. The SEM is used to locate
breaks in microcircuits, examine metals
for fatigue and stress fractures, and in the analysis of air and water pollution.
New uses for electron microscopes are found every year, as these remarkable
instruments constantly expand our view of the world.
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electrons in a transmission electron microscope (TEM).
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