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 low-mass 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 cathode-ray 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.
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.
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.
See also Cyclotron; Field ion microscope; Geiger counter; Mass spectrograph; Neutrino detector; Scanning tunneling microscope; Synchrocyclotron; Tevatron accelerator; Ultramicroscope.
