A microscope that uses electrons instead of visible light to
produce highly magnified images of objects. Scientists use electron microscopes
in many different fields of research, including medicine, biology, chemistry,
metallurgy, entomology (the study of insects), and physics. Since its
introduction in the 1930s, the electron microscope has revolutionized the study
of microscopic structures and surfaces. See also Microscope.
Microscopes can only resolve structures that are larger than
the length of the waves (such as light waves) reflecting off of them. Electron
microscopes are able to obtain much higher powers of magnification than
standard visible light microscopes because electrons have much shorter
wavelengths associated with them than light waves. The highest magnification
achievable with light microscopes is about 2,000X (times); modern electron
microscopes can achieve magnifications approaching 1,000,000X.
Their invention
The invention of the electron microscope was made possible
by a number of theoretical and experimental advances in physics and
engineering. The main concept on which the electron microscope is founded—that
electrons have a wavelike nature—was hypothesized by French physicist Prince
Louis Victor de Broglie in 1923 (see Quantum Theory). In 1927, de Broglie’s
hypothesis was experimentally verified by American physicists Clinton J.
Davisson and Lester H. Germer, and independently by English physicist George
Paget Thomson. In 1932 German engineers Max Knoll and Ernst Ruska built the
first transmission electron microscope. In 1938 Ruska and German engineer Bodo
von Borries built the first model of the commercial TEM for the Siemens-Halske
Company in Berlin, Germany. The English engineer Sir Charles Oatley invented the
SEM in its present form in 1952.
Powerful research tools
Electron microscopes have proven powerful research tools for
investigating the basic structure of matter, specifically in the fields of
biology and solid-state science. They have, for example, helped to reveal the
surface structures of various materials and confirmed the shapes and behaviors
of bacteria as well as animal and human cells and cell components. They are
important in research examining the effects of various manipulations or treatments
of these various types of subject matter. Scientists and publishers often add
color to the highly detailed electron micrographs to increase interest, to help
distinguish portions of the image, and to highlight important areas. Electron
microscopes have given scientific and lay media remarkable pictures, such as
the “faces” of insects, the shapes of microscopic organisms, and the surface
structure of molecules of new, high-tech alloys and other substances. They are
also becoming important to regular clinical pathology at medical centers.
Two main types
There are two main types of electron microscope, the
transmission electron microscope (TEM) and the scanning electron microscope
(SEM).
TEM type
The TEM consists of an electron source, a number of lenses,
and a system that projects an image onto a fluorescent screen or photographic
plate. The lenses must be electric or magnetic lenses because standard glass
lenses for light microscopes will not focus a beam of electrons. The electron
source is a filament of tungsten that releases electrons when heated. A high
voltage electric field (50,000 to 100,000 volts) applied between a pair of
metal plates accelerates the electrons, and electric or magnetic lenses
condense the electrons into a narrow beam. The electron beam then passes
through the specimen and individual electrons are scattered in various
directions depending upon the density of the material they encounter. Denser
material scatters the electrons more so that fewer of them reach the device
used to detect the electrons. Once the electrons pass through the specimen, the
objective lens focuses them using magnetic fields. Specimens must be sliced
extremely thin for use in a TEM because only electrons that pass through the
specimen are recorded. Modern TEMs are capable of magnifications of between
1,000X to about 1,000,000X.
SEM Type
The SEM works by scanning a tightly focused electron beam
over a sample. Electrons in the beam scatter off of the sample and onto a
cathode ray tube, or screen. Each point on the sample corresponds to a pixel or
picture element on the screen. The more electrons that hit a particular element
of the screen, the brighter the pixel appears. As the electron beam scans over
the entire sample, a complete image of the sample is displayed on the monitor.
SEMs have a range of magnification of between 20X to 200,000X.
The SEM is particularly useful because it can produce
detailed three-dimensional images of the surface of objects, whereas a TEM can
only produce two-dimensional images. Samples scanned by an SEM do not need to
be thinly sliced, as do TEM specimens, but they must be dehydrated to prevent
electrons from being scattered by water molecules in the sample. The vessels
that house electron microscopes must be evacuated to very high vacuum to
prevent the scattering of the electrons off of air molecules. (Adapted from
‘Encarta Encyclopedia’)