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November 1959 Popular Electronics[Table of Contents]People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Popular Electronics was published from October 1954 through April 1985. All copyrights are hereby acknowledged.
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 beam.
By Morris M. Rubin
Long after optical microscopes have reached their limit, the electron microscope goes on magnifying ... up to 200,000 times.
Since the 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.
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.
Whereas 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.
The enlarged 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 the microscope.
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)
1. (a); total magnification 160,000X; courtesy of Philips
2. (c); total magnification 425,000X; courtesy of College de France and RCA
3. (c); 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
Posted May 23, 2014