The Amazing Electron Microscope
November 1959 Popular Electronics
Optical magnification is only useful to the point where resolution is limited by the wavelength of light
representing the object under observation. Astronomer William Dawes first provided a means of
quantification based on an ability to visually resolve closely spaced stars. Known as the Dawes Limit, a
value of 4.56/D arc seconds was empirically determined (D is aperture of instrument in inches). A
theoretical upper limit to magnification of any optical system with perfect optics is around 2,000. The
electron microscope removed that resolving limit by shooting a stream of electrons with radii much less
than the wavelength of visible light, and measuring its reflection. Images are necessarily in 'false
color' because we cannot perceive the real wavelength/color of the surface revealed by the electron
of Contents]People old and young enjoy waxing nostalgic about and learning some of the history of early electronics.
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Amazing Electron Microscope
By Morris M. Rubin
Long after optical microscopes
have reached their limit, the electron microscope goes on magnifying ... up to 200,000 times.
time of Anton von Leeuwenhouk - the first great microscope designer - scientists have relied on the microscope as
one of their key tools. Year after year, as techniques for manufacturing optical glass were improved, new and better
microscopes enabled scientists to see increasingly minute objects. Then, about 1890, it seemed that light microscopes
had come to the end of the line. Beyond about 2000 times magnification, even the finest, most perfectly designed
microscope showed only a fuzzy blob.
Blocking the development of more powerful microscopes was a basic characteristic
of light itself. Similar to sound, light travels in waves of measurable length. In the middle of the visible light
spectrum, for example, the waves have a length of about 6/250,000 of an inch. In order for light waves to differentiate
between two points of an object, they must be separated by about one-third the length of a light wave, or about
2/250,000 of an inch. Objects smaller than about half a wavelength cannot be magnified clearly by a light microscope,
no matter how perfect its lenses.
Scientists reasoned that since the basic difficulty was caused by the
relatively long wavelengths of "ordinary" light, if it were possible to employ some type of light which had shorter
wavelengths, greater useful magnification could be achieved. This possibility was explored, and by utilizing ultraviolet
light (which has a wavelength only one-third that of visible light), microscopes were designed which could magnify
up to 5000 times, over double the limit of visible-light microscopes.
At this point, the light microscope
reached the zenith of its design capacities. If scientists wanted more magnification, they had to find a new way
of getting it.
Electrons to the Rescue. The theory of the electron microscope was suggested in the 1920's.
Experiments showed that electrons acquired a measurable characteristic wavelength when they were speeded up by subjecting
them to high-voltage fields. The higher the voltage, the greater was the velocity of the electrons, and the shorter
became the apparent wavelength. Further, it had been proven that electrons could be bent or refracted by magnetic
fields in much the same way light is bent and refracted by optical lenses.
Comparison of electron microscope and ordinary light microscope. The basic principles are the same, but the
electron microscope uses coils of wire to deflect and focus an electron beam magnetically rather than glass
lenses to bend and refract light.
Two of the most widely used electron microscopes are shown here. At left is the RCA EMU-3, capable of 200,000X
magnification. At right is the Norelco EM100B, which magnifies to 90,000 times.
Co-inventor of the first RCA electron microscope, Dr. James Hillier, is shown at left with RCA's Model EMB,
marketed in 1940.
A 1959 Russian unit exhibited at the recent Soviet Exhibition in New York.
Therefore, it seemed logical that light, the limiting factor in the process of magnification, could be replaced
by a stream of electrons which would have a much shorter wavelength and so would allow greater magnification. With
this important concept under their hats, scientists began work on the design of an electron microscope.
By the late 1930's, experimental microscopes were in operation in Europe, Canada, and the United States. And then,
in 1940, RCA marketed the first commercial American electron microscopes. These initial instruments, though crude
by present-day standards, were fantastically superior to the best light microscopes ever produced.
even an ultraviolet microscope was limited to a magnification of 5000 times, these early electron microscopes were
capable of enlargements of 100,000 times. Today's models magnify more than 200,000 times - enough to see an object
one-millionth the diameter of a human hair - and by enlarging the image still further by photographic means, useful
magnifications of over one million diameters are possible!
Electrons Replace Light. Similar in principle
to light microscopes, the electron microscope uses a series of lenses to magnify the specimen in a step-by-step
process. But while a light microscope uses glass lenses to bend light rays, the electron microscope's "lenses" are
coils of wire - similar to the deflection coils of a television set - which bend and deflect a stream of electrons.
Electrons emitted by the electron gun pass through the condenser lens which concentrates the beam of electrons
on the specimen. Since the specimen has been sliced so thin as to be partially transparent, the electrons pass through
it in varying numbers depending on the density of the specimen at anyone point. Thus a pattern of varying electron
densities is produced.
Although this pattern is invisible to the eye, it could be shown by placing a fluorescent
screen below the specimen. In practice, however, the electrons pass through the objective lens, which provides the
first step of magnification. Just before they reach the projector lens, a "spread-out" representation of the density
pattern is formed, the center area of which is then further magnified by the projector lens.
specimen can be viewed directly on a fluorescent screen (which looks and works like a television screen), or the
image can be photographed by special cameras - usually built right into the electron microscope. Enlargement of
the resulting photographs allows further magnification of the specimen.
Price Tags. In addition to its optical
system, an electron microscope must have an ultra-stable high-voltage supply and a highly efficient vacuum system.
This complexity accounts for the big price tags on today's electron microscopes - from $12,000 to $40,000, depending
on the magnification desired, the make, etc.
Norelco (Philips of Holland) and RCA are the largest producers
of these units. Also active in the field are manufacturers in Germany and Japan. And the Russians have gotten into
the act, too, by producing an electron microscope which seems to be an adaptation of a 1940 RCA model.
Limitations. Useful though the electron microscope may be, it still has its limitations. Since the high-voltage
electrons are fatal to living organisms, the electron microscope cannot be used to view live bacteria, viruses,
etc. Also, the beam of electrons can't penetrate more than 1/25,000-of-an-inch, so the electron microscope cannot
be used to view objects that are any thicker - the wing of a fly, for example.
The solution to the latter
problem has been the development of special devices that can carve off slices of the object to be viewed which are
thin enough to allow the passage of electrons. It's easy to see how such a "slicer" works with softer materials,
but how do we go about slicing off a layer of steel 1/25,000-of-an-inch thick? The answer to this question is surprisingly
simple. A "replica" of the steel's surface is made on a soft material - such as wax. The replica is easy to slice
and, when it is mounted on an exceedingly thin transparent membrane, it takes the place of the original object in
Importance. About one thousand electron microscopes are now in use in laboratories across
the nation. While they are invaluable tools in research aimed at finding the causes of diseases, especially cancer,
they are also useful in solving a wide variety of industrial problems. For example, the wearing qualities of rubber
tires can be judged by careful examination of an electron microscope photograph, thus eliminating the need for long
and tedious road tests.
But it is in the study of cells that the electron microscope finds its most exciting
application. Cells are grown, nourished, and reproduced through a process of protein synthesis. With the aid of
electron microscopes, scientists have been able to see these processes - which are truly the "secrets of life" -
for the first time.
Man is an insatiably curious creature. One of his most efficient means of satisfying
his hunger for knowledge and understanding is the electron microscope.
Can You Identify These Pictures?
All were taken with the aid of an electron microscope
(answers are at bottom of page)
(a) metal surface
(b) human skin
(d) wool rug
(a) wing of a fly
(b) moon craters
(c) protein molecule
(d) cellulose fiber
(a) lead carbonate
(c) nucleic acid
(d) cotton balls
(a) an ant's toe
(b) polio virus
a sloppy knot
(d) limesoap grease
(a) Sahara Desert
(b) silk fiber
Monday morning at Coney Island
1. (a); total magnification 160,000X; courtesy of Philips
2. (c); total magnification 425,000X; courtesy of College de France and RCA
total magnification 112,000X; courtesy of Dr. C. E. Hall, M.I.T.
4, (d); total magnification 68,000X; courtesy
of Esso Research & Engrg. Co.
5. (c); total magnification 14,680X; courtesy of Dow Chemical Co, and RCA