Electron microscope are the magnifying instruments that utilizes the beam of high speed electrons to illuminate the object. Electron microscopy was discovered for visualizing the ultra-structure of the microorganisms, cells etc. It gives high magnification (up to 1000000x) and incredibly high resolution (dependent upon the wavelength of radiations).
Electron microscopes use signals arising from the interaction of an electron beam with sample to obtain information about structure, morphology and composition.
It uses electron beam to form the image of the object and magnification is obtained by electromagnetic fields. However, in light microscopes, light waves are used to produce the image and magnification is obtained by a system of optical lenses.
The first transmission electron microscope was developed by Ernst Ruska and Max Knoll of Germany in 1931.
The general principle of electron microscope is analogous to light microscope with the difference being use of electron beams to analyze the specimen instead of visible light. Electron gun or white hot tungsten filament emits high velocity electrons when high voltage [(40,000- 10,00,000) volts] is passed through it. Electrons (negatively charged) pass through hole in anode forming an electron beam, which is focused by condenser lens onto specimen.
Electrons are differentially imparted by various structures. Some of the electrons are scattered and some are absorbed by atoms of specimen. Electrons which pass through specimen are focused with series of magnetic objective lens onto photographic plate or fluorescent screen on which electrons interact as like photons and generate an image of the specimen.
The lenses used are magnetic coils capable of focusing the electron beam on the specimen and illuminating it. The strength of the magnetic lens depends upon the current that flows through it. Greater the flow of the current, greater will be strength of the magnetic field.
They are used to observe the ultra microscopic structure of various microorganisms. Use of Electron microscopy has been a greater aspect in development of immunological and molecular biology.
Electron microscopes can be of two types:
- Transmission electron microscope
- Scanning electron microscope