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. 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 0.004 nanometre (see de Broglie wavelength) 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, 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. 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.
Principle of transmission electron microscope
- statistical mechanics
- statistics
- stator
- steady-state theory
- steam distillation
- steam engine
- steam point
- steel
- Stefan's law
- stellar evolution
- stellar wind
- steradian
- stere
- stimulated emission
- Stirling engine
- STM
- stochastic process
- stokes
- Stokes' law
- Stokes' parameters
- Stokes' theorem
- Stokes, Sir George Gabriel
- stop
- stopping power
- storage ring