Electron microscope

The invention: 



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

its wavelength.

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

their limit.





Matter Waves



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,

was accurate.

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.”





Impact



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 Ruska





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;

Electron microscope.





Further Reading