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picture tube, television pictures, electron microscopes, wavelength of visible light, elementary particles

In addition to using electrons for electrical devices, manufacturers use beams of pure electrons to produce television pictures and X rays, and to illuminate objects in electron microscopes. The electron beam for each of these devices is created by heating a cathode, a negatively charged metal that emits electrons. The electrons accelerate as they are attracted to the anode, a positively-charged piece of metal.

Electron beams are used in the cathode-ray tube (or picture tube) of traditional television screens. In the cathode-ray tube, the electrons race toward a hollow anode so that a narrow, fast beam of electrons shoots out through the hole in the anode. The higher the positive charge on the anode, the greater the speed—and thus the energy—of the beam. The tube must be emptied of air to prevent the electrons from being slowed or scattered by collisions with air molecules. The beam of electrons is focused so that it hits a specific spot on the television screen, which is covered with luminescent material. When the electrons hit this material, they excite its atoms. The excited atoms then lose this extra energy by releasing flashes of light. A changing electromagnetic field inside the picture tube affects the negatively charged electrons and makes the electron beam rapidly scan across the screen, moving horizontally and vertically. The flashes caused by the beam build up a continually changing picture.

When a high-powered electron beam hits a metal anode, it can create X rays for medical or industrial purposes. A fast-moving electron can knock an inner-shell electron out of an atom. As an outer-shell electron jumps inward to fill the inner-shell vacancy, the atom emits an X ray, a high-energy photon invisible to the eye. X rays are absorbed by heavier atoms, such as those in bones, but pass through lighter atoms, such as those in flesh. X rays can also react with chemicals in specialized film to create a picture (see Photography). If a patient’s arm is placed in front of a photographic film, exposing the arm to an X-ray beam will create an image of the bone on the film.

Scientists use powerful X rays created by electrons to probe the structure of atoms and molecules. They produce these X rays by accelerating a beam of electrons, confined by magnets in a circular tube, to a very high energy. Higher and higher energy electrons release radiation with shorter and shorter wavelengths, in this case, X rays. The shorter the wavelength, the finer the detail the X rays reveal.

While scientists usually describe the electron as a particle, the electron can also behave like a wave. Scientists use this aspect of electron behavior to illuminate extremely small objects. Ordinary light can only resolve objects that are larger than the wavelength of the light waves illuminating them. For smaller objects, the light waves scatter randomly off the object and do not reveal its shape. The wavelength of visible light is about a millionth of a meter. Electrons can have smaller wavelengths than visible light and thereby reveal objects many times smaller. Electron microscopes, using beams of electrons instead of light, can create images of objects, such as viruses, too small to be visible by ordinary microscopes. Electron energies are usually measured in electron volts (eV), where 1 eV is the energy acquired by an electron when it is accelerated in a vacuum by 1 volt. Physicists can use electrons they’ve accelerated to very high energies (giga-electron volts, or 109 eV, which is 1 billion electron volts) to reveal elementary particles such as protons, neutrons, and even quarks.

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