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Electron microscope

Electron microscope, microscope that uses beams of electrons to produce extremely high magnifications. The optical microscope cannot produce images of objects smaller than the wavelength of the light used. But when the French physicist Victor De Broglie discovered in 1924 that electrons could behave like waves, it became apparent that streams of electrons could be manipulated to produce magnified images. A wavelength 100,000 times shorter than that of green light could be produced, making enormous magnifications possible. In 1935 an instrument was produced that exceeded the resolution of the optical microscope. An electron gun, consisting of a hot tungsten filament and electrodes carrying up to 100,000 volts, generates a stream of high-velocity electrons. This electron beam is controlled by a system of magnetic fields generated by circular coils that are analogous to the lenses in ordinary microscopes. The beam has to travel in a high vacuum (about 1/10,000,000 atm) to avoid scattering the electrons and blurring the image. The object to be examined must be extremely thin to allow the passage of the electrons. On passing through the specimen, the electrons are scattered to varying extent by the different atoms in it. The scattered electrons produce contrast in an image either on a photographic plate or on a fluorescent screen. Further magnification can be obtained by enlarging the photograph. Instrumental magnifications up to 200,000 are common, and with photographic enlargements, magnifications exceeding 2 million can be made. On May 20, 1970, Albert V. Crewe of the University of Chicago succeeded in taking pictures of individual uranium and thorium atoms in organic compounds. He has also taken remarkably fine pictures of DNA, the complex helical molecule that carries genetic information in almost all living organisms.

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