Physics · Ch 7 — Dual Nature of Radiation and Matter
Electron Microscope
Electron Microscope
The resolving power of any microscope -- its ability to distinguish two closely spaced points as genuinely separate -- is inversely proportional to the wavelength of the radiation used to illuminate the object under study; a shorter illuminating wavelength buys both a higher resolving power and a higher achievable magnification. Because an electron's de Broglie wavelength (typically a few picometres to a few angstroms, depending on the accelerating voltage) is many thousands of times shorter than the wavelength of the visible light used in an ordinary optical microscope, replacing light with a beam of electrons -- the direct, practical application of the wave nature of matter -- makes it possible to build a microscope with far higher resolving power and far higher magnification than any optical instrument. Such an instrument is called an electron microscope; electron microscopes achieving magnifications well beyond 2,00,000 times are routine in modern research laboratories.
An electron microscope's overall construction and working closely parallel an optical microscope's, component for component, with the illuminating radiation and the focusing elements swapped out. In an optical microscope, a light source illuminates the object through a condenser lens, the object is imaged by an objective lens, an intermediate image forms, and a projector lens produces the final magnified image on a viewing screen. In an electron microscope, an electron source replaces the light source, and glass lenses are replaced throughout by electrostatic or magnetic lenses -- the electron beam, in passing through a suitably arranged electric or magnetic field, is made to converge or diverge exactly as a glass lens converges or diverges a light beam, giving precise control over focusing. Electrons emitted from the source are first accelerated by a high potential and made into a roughly parallel beam by a magnetic condenser lens; as this beam passes through the sample being studied, it picks up the sample's image information, an …
What this figure shows. Panel (a) shows the familiar layout of an optical microscope: a light source, a condenser lens, the object being viewed, an objective lens, a projector lens, and finally a viewing screen, with an intermediate image formed partway along the light path. Panel (b) shows the directly analogous layout of an electron microscope: an electron source in place of the light source, magnetic condenser and objective and projector lenses in place of the glass lenses, the object again positioned partway along, an intermediate image formed midway, and a photographic plate in place of the viewing screen at the end. Panel (c) is a photograph of an actual electron microscope instrument. Together the three panels emphasise that an electron microscope's overall architecture mirrors an optical microscope's exactly, component for component, with only the illuminating radiation (electrons instead of light) and the focusing elements (magnetic/electric lenses instead of glass) actually changed -- and this s …