electron microscope

electron microscope Microscope used for producing an image of a minute object. It ‘illuminates’ the object with a stream of electrons, and the ‘lenses’ consist of magnets that focus the electron beam. Smaller objects can be seen because electrons have shorter wavelengths than light, and thus provide greater resolution. The image is obtained by converting the pattern (made by electrons passing through the object) into a video display, which may be photographed. These microscopes can magnify from 2000 to a million times.

electron microscope n. a microscope that uses a beam of electrons as a radiation source for viewing the specimen. The resolving power (ability to register fine detail) is a thousand times greater than that of an ordinary light microscope.

electron microscope A form of microscope that uses a beam of electrons instead of a beam of light (as in the optical microscope) to form a large image of a very small object, such as a cell organelle, a virus, or a DNA molecule. In optical microscopes the resolution is limited by the wavelength of the light. High-energy electrons, however, can be associated with a considerably shorter wavelength than light; for example, electrons accelerated to an energy of 10⁵ electronvolts have a wavelength of 0.04 nanometre, enabling a resolution of 0.2–0.5 nm to be achieved. The transmission electron microscope (see illustration) has an electron beam, sharply focused by electron lenses (coils producing a magnetic field or electrodes between which an electric field is created), passing through a very thin metallized specimen (less than 50 nanometres thick) onto a fluorescent screen, where a visual image is formed. This image can be photographed. The scanning electron microscope can be used with thicker specimens and forms a perspective image, although the resolution and magnification are lower. It is used particularly for examining surface features of small objects, such as pollen grains. In this type of instrument a beam of primary electrons scans the specimen and those that are reflected, together with any secondary electrons emitted, are collected. This current is used to modulate a separate electron beam in a TV monitor, which scans the screen at the same frequency, consequently building up a picture of the specimen. The resolution is limited to about 10–20 nm. See also field-emission microscope; field-ionization microscope.