Transmission Electron Microscopy

Transmission Electron Microscopy:

The basic principles of electron microscopy are identical to those of light microscopy; the main difference is that in electron microscope electromagnetic lenses are used not optical lenses. It also focuses a high velocity electron beam rather than visible light. The electrons are absorbed by atoms in air and that is the reason why the whole tube between the electron source and the viewing screen is sustained beneath an ultrahigh vacuum.

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Figure: Working principle of light and electron microscope

The TEM directs a beam of electrons via a specimen. The electrons are emitted by a tungsten cathode whenever it is electrically heated. A condenser lens concentrates the electron beam on to the sample, objective and projects them on to a presentation screen or on a piece of photographic film.

The smallest distance D at which the two objects can be differentiated is proportional to the wavelength λ of the light which illuminates the objects. Therefore the limit of resolution for the electron microscope is hypothetically 0.005 nm or 40,000 times better than the resolution of the light microscope and 2 million times enhanced than that of unaided human eye. However in reality a resolution of 0.10nm can be acquired with TEM, around 2000 times better than the resolution of the light microscopes.

 

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