The electron microscope has done for the very small what the telescope did for the vastness of space - revealed previously unseen worlds. The theory behind the electron microscope was proposed in 1924, and the first practical one was built in Germany in 1932. The first electron microscope in North America was built at the University of Toronto in 1938. While a light microscope magnifies about 1600 times - enough to see bacteria 1/5000 cm long - the Centre's research electron microscope can magnify an incredible 500,000 times on the fluorescent viewing screen. The normal working magnification ra~ges from 2,800 to 90,000 times­ powerful enough to see tiny viruses one millionth of a centimetre across. But magnification isn't the only criterion in microscope performance. Equally important is resolving power. This is the ability to distinguish two points in an object as being separate from each other. Your eye, for ex­ ample, has a resolving power of one hundredth of a centimetre - that is, it can only distinguish between points that are at least one hundredth of a centimetre apart. If two points are closer than this, they will be seen as a single point, no matter how great the magnification may be. o In microscopy, resolving power is expressed in angstroms (A). One angstrom is 1/10P,000,000 cm. The best light microscope has a resolving power of 2000 A (1/50,000 cm.), which is 500 times greater than the eye. ~ut the best modern electron microscope has a resolving power of only 2 A ­ 1,000 times better than the best light microscope, and 500,000 times greater than the naked eye! Electron microscopes have this greater resolving power because they use electrons, which have shorter wavelengths than visible light. Although no one has yet been able to produce a microscope with sufficient magnifying and resolving power to see a single atom, the electron micro­ scope can photograph molecules, which are groups of atoms. 'JA~ CANADIANS IN PHYSICS

University of Toronto have a resolving power more than 10000 times Graduate Students Make History greater than light microscopes. In practice, however, electron microscopes have resolving Although many research groups around the world powers about 1000 times greater than light were attempting to design and build electron microscopes. microscopes in the 1930s, the first high-resolution The diagram shows the typical design of a electron microscope that was practical and there­ transmission electron microscope. The barrel of fore became the prototype for the first commercial the microscope must be evacuated, because instrument was designed, built, and tested by two electrons would be scattered by molecules in the graduate students at the University of Toronto. air. Electrons would not penetrate glass lenses, of James Hillier and Albert Prebus are shown in the course, so focussing is accomplished by magnetic photograph with the electron microscope that they fields created by electromagnets. These magnetic built in 1938. Hillier continued to perfect and use "lenses" do not have to be moved or changed, the electron microscope while he completed his because their focal lengths can be changed simply Ph.D. degree. In 1940, Hillier joined the staff of the by adjusting the magnetic field strength of the Radio Corporation of America (RCA) in Camden, electromagnets. Since electrons cannot penetrate New Jersey, where he continued to improve the glass, the extremely thin electron microscope electron micro­ specimens are placed on a wire mesh so that the scope. In 1969, electrons can penetrate the areas between the H~lier became tiny wires. the executive vice president in .-----'0/ hot filament charge of research power source ~ anode and engineering for RCA. In this \: condenser position, he was "lens" responsible for /[!; I' all of the research, electro- ri0 I --~-- specimen development, magnets --~ ~ - objective "lens" and engineering ~ , programs. '"'" !/ 'j- ~--- first image The race to build electron microscopes was [!;- " -- . +-- "/-~- projector (ocular) based on Davisson and Germer's verification of the ~ I "lens" wave properties of electrons. Electron microscopes / ;"\, have much greater resolving power than light t _." _____I '",,­... final image microscopes, due to their very short wavelengths. Resolving power is the ability to distinguish two The photograph at the beginning of this chapter or more objects as separate entities, rather than was produced by a scanning electron microscope. as one large object. If the distance between two These instruments function on a very different objects is much less than the wavelength, a micro­ principle than do transmission electron micro­ scope "sees" them as one particle, rather than as scopes. A very tiny beam of electrons sweeps back two. You can magnify the image to any size, but all and forth across the specimen, and electrons that that you will see is one large, blurred object. Since bounce back up from the sample are detected. the shortest wavelength of visible light is about Scanning electron microscopes were first devel­ 400 nm and electrons can have wavelengths of oped in 1942, but they were not commercially 0.005 nm, electron microscopes could theoretically available until 1965.

McGraw·Hill Ryerson Physics 12 TRANSMISSION ELECTRON MICROSCOPE There are two types of electron microscopes. The earliest, and most common, is the transmission elec­ tron microscope (TEM). Using electromagnetic lenses, it focuses a beam of electrons which is ~\-f--- Insulator transmitted through an extremely thin specimen into another series of electromagnetic lenses_ These -(---1---- Electron enlarge the specimen image carried Gun by the beam and project it onto a fluorescent screen where it can be seen. Alternatively, the image can Condenser be trained onto a photographic Lenses plate or 35 mm film to obtain a permanent record of the image.

Specimens examined in the TEM must be very thin, so that the ,,,,....+If--- Objective electron beam can penetrate them_ Lens To prepare biological samples for this microscope, specimens are E?I~~ttrtttl-.-- Specimen embedded in hard plastic, then Position sliced by a diamond or glass blade into sections that are only a few hundred angstroms thick. The delicate slices are floated off the knife edge onto water and are picked up on a thin copper grid, 3 mm across. The specimen is then stained with heavy metals (such as uranium or lead) to improve contrast among its var­ ious parts. The grid with its stained specimen is inserted into the TEM's vacuum chamber, where it is struck by the beam.

The TEM works on the same principles as the light micro­ scope, except that it uses electrons instead of light to produce an image, and magnetic lenses instead of glass lenses to focus the beam.

Transmission Electron Microscope SCANNING ELECTRON MICROSCOPE With the advent of the scanning electron microscope (SEM) the three dim­ ensional appearance of microscopic objects could finally be seen. The SEM,. however, can only be used to observe surface features because electrons that pass through the specimen are not seen. The SEM was developed in 1938 by M. vonArdenne, but the first commercial model was introduced only in 1965.

The SEM works in a similar way to a television picture tube. The micro­ scope's condenser lenses focus the electron beam into a fine ray that scans the surface of a specimen, (just as an electron beam moves back and forth across the face of a television tube.) As electrons strike the specimen in the microscope, they are scattered ----i--- Electron Gun or knock secondary electrons from the sample. The scattered and sec­ ondary electrons are picked up by a detector and transmitted onto • --- -t-- Electron Beam a cathode-ray viewing screen, like a television set. Crevices in a specimen produce fewer detect­ able electrons whereas projections are highlighted. The result is an image with three-dimensional appearance.

Because the SEM doesn't need thin sections of specimens, it can deliver pictures of whole organisms, from protozoa to in­ sects. Samples are covered with a thin coating of precious metals before being exam­ ined, to sharpen the image. The resolution of the SEM is much better than the light microscope, but poor­ er than the TEM. Commer­ cial instruments usually operate at 100 A(one millionth of a centimetre.) Vacuum System Opening of the fallopian tube near the ovaries 5177X Scanning Electron Microscope

USES OF ELECTRON MICROSCOPES In medicine, electron micro­ scopes are used to study cell­ ular changes in diseases such as cancer, and to diagnose blood and viral diseases, var­ ious types of muscular dys­ trophy and kidney disorders.

The TEM is used in dark field microscopy to view the struc­ ture of large molecules such as DNA. The SEM is used to locate breaks in microcircuits, examine metals for fatigue and stress fractures, and in the analysis of air and water pollution. New uses for electron microscopes are found every year, as these remarkable instruments constantly expand our view of the world. ·~ ;. ==~--:: ""-'-':"''':'':C; ·~ · ~,7::'';~':::: ';:;~= '''== :==:", =~==-=;:: :;. ; ;; :·c.'C·:O·; ;:;:-"::":. =:,, ·=;:,:~:::;.··=~-,",-,,",,· .o ·. "" "'-' " .... . j' , .. '.--. ~. -- ' . o;,~.~ ~ LM TEM G ~i [)rf ~ V fj n c:::::;.: . 1_ kl N ~L.! -;- ~mp "Illumination" Electrons ..-7 M

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