18 June 2009
A device for viewing extremely small objects that
uses electron beams and “electron lenses” instead of the light
rays and optical lenses used by ordinary microscopes.
The people behind the invention:
Ernst Ruska (1906-1988), a German engineer, researcher, and
inventor who shared the 1986 Nobel Prize in Physics
Hans Busch (1884-1973), a German physicist
Max Knoll (1897-1969), a German engineer and professor
Louis de Broglie (1892-1987), a French physicist who won the
1929 Nobel Prize in Physics
Reaching the Limit
The first electron microscope was constructed by Ernst Ruska
and Max Knoll in 1931. Scientists who look into the microscopic
world always demand microscopes of higher and higher resolution
(resolution is the ability of an optical instrument to distinguish
closely spaced objects). As early as 1834, George Airy, the eminent
British astronomer, theorized that there should be a natural limit to
the resolution of optical microscopes. In 1873, two Germans, Ernst
Abbe, cofounder of the Karl Zeiss Optical Works at Jena, and Hermann
von Helmholtz, the famous physicist and philosopher, independently
published papers on this issue. Both arrived at the same
conclusion as Airy: Light is limited by the size of its wavelength.
Specifically, light cannot resolve smaller than one-half the height of
One solution to this limitation was to experiment with light, or
electromagnetic radiation, or shorter and shorter wavelengths.
At the beginning of the twentieth century, Joseph Edwin Barnard
experimented on microscopes using ultraviolet light. Such instruments,
however, only modestly improved the resolution. In
1912, German physicist Max von Laue considered using X rays.
At the time, however, it was hard to turn “X-ray microscopy” into
a physical reality. The wavelengths of X rays are exceedingly
short, but for the most part they are used to penetrate matter, not
to illuminate objects. It appeared that microscopes had reached
In a new microscopy, then, light—even electromagnetic radiation
in general—as the medium that traditionally carried image information,
had to be replaced by a new medium. In 1924, French
theoretical physicist Louis de Broglie advanced a startling hypothesis:
Matter on the scale of subatomic particles possesses wave
characteristics. De Broglie also concluded that the speed of lowmass
subatomic particles, such as electrons, is related to wavelength.
Specifically, higher speeds correspond to shorter wavelengths.
When Knoll and Ruska built the first electron microscope in 1931,
they had never heard about de Broglie’s “matter wave.” Ruska recollected
that when, in 1932, he and Knoll first learned about de
Broglie’s idea, he realized that those matter waves would have to be
many times shorter in wavelength than light waves.
The core component of the new instrument was the electron
beam, or “cathode ray,” as it was usually called then. The cathoderay
tube was invented in 1857 and was the source of a number of
discoveries, including X rays. In 1896, Olaf Kristian Birkeland, a
Norwegian scientist, after experimenting with the effect of parallel
magnetic fields on the electron beam of the cathode-ray tube, concluded
that cathode rays that are concentrated on a focal point by
means of a magnet are as effective as parallel light rays that are concentrated
by means of a lens.
From around 1910, German physicist Hans Busch was the leading
researcher in the field. In 1926, he published his theory on the
trajectories of electrons in magnetic fields. His conclusions confirmed
and expanded upon those of Birkeland. As a result, Busch
has been recognized as the founder of a new field later known
as “electron optics.” His theoretical study showed, among other
things, that the analogy between light and lenses on the one hand,
and electron beams and electromagnetic lenses, on the other hand,
Beginning in 1928, Ruska, as a graduate student at the Berlin Institute
of Technology, worked on refining Busch’s work. He found
that the energy of the electrons in the beam was not uniform. This
nonuniformity meant that the images of microscopic objects would
ultimately be fuzzy. Knoll and Ruska were able to work from the
recognition of this problem to the design and materialization of a
concentrated electron “writing spot” and to the actual construction
of the electron microscope. By April, 1931, they had established a
technological landmark with the “first constructional realization of
an electron microscope.”
The world’s first electron microscope, which took its first photographic
record on April 7, 1931, was rudimentary. Its two-stage total
magnification was only sixteen times larger than the sample. Since
Ruska and Knoll’s creation, however, progress in electron microscopy
has been spectacular. Such an achievement is one of the prominent
examples that illustrate the historically unprecedented pace of
science and technology in the twentieth century.
In 1935, for the first time, the electron microscope surpassed
the optical microscope in resolution. The problem of damaging
the specimen by the heating effects of the electron beam proved
to be more difficult to resolve. In 1937, a team at the University of
Toronto constructed the first generally usable electron microscope.
In 1942, a group headed by James Hillier at the Radio Corporation
of America produced commercial transmission electron
microscopes. In 1939 and 1940, research papers on electron microscopes
began to appear in Sweden, Canada, the United States,
and Japan; from 1944 to 1947, papers appeared in Switzerland,
France, the Soviet Union, The Netherlands, and England. Following
research work in laboratories, commercial transmission electron
microscopes using magnetic lenses with short focal lengths
also appeared in these countries.
Ernst August Friedrich Ruska was born in 1906 in Heidelberg
to Professor Julius Ruska and his wife, Elisabeth. In 1925
he left home for the Technical College of Munich, moving two
years later to the Technical College of Berlin and gaining practical
training at nearby Siemens and Halsk Limited. During his
university days he became interested in vacuum tube technology
and worked at the Institute of High Voltage, participating
in the development of a high performance cathode ray oscilloscope.
His interests also lay with the theory and application of electron
optics. In 1929, as part of his graduate work, Ruska published
a proof of Hans Busch’s theory explaining possible lenslike
effects of a magnetic field on an electron stream, which led
to the invention of the polschuh lens. It formed the core of the
electron microscope that Ruska built with his mentor, Max
Kroll, in 1931.
Ruska completed his doctoral studies in 1934, but he had already
found work in industry, believing that further technical
development of electron microscopes was beyond the means of
university laboratories. He worked for Fernseh Limited from
1933 to 1937 and for Siemens from 1937 to 1955. Following
World War II he helped set up the Institute of Electron Optics
and worked in the Faculty of Medicine and Biology of the German
Academy of Sciences. He joined the Fritz Haber Institute
of the Max Planck Society in Berlin in 1949 and took over as director
of its Institute for Electron Microscopy in 1955, keeping
the position until he retired in 1974.
His life-long work with electron microscopy earned Ruska
half of the 1986 Nobel Prize in Physics. He died two years later.
To honor his memory, European manufacturers of electron microscopes
instituted the Ernst Ruska Prizes, one for researchers
of materials and optics and one for biomedical researchers.
See also: Cyclotron; Field ion microscope; Geiger counter; Massspectrograph;
Neutrino detector; Scanning tunneling microscope;Synchrocyclotron;