Electron Microscope

The development of the electron microscope was based on theoretical work done by Louis De Broglie, who found that wavelength is inversely proportional to momentum. A few years later to De Broglie, the first modern prototype electron microscope is introduced by Max Knoll and Ernst Ruska.

An electron microscope is type of microscope that uses a beam of electrons, rather than electromagnetic radiation, to "illuminate" the specimen and create an image of a specimen. It has much higher magnification and resolving power than a light microscope, with magnifications up to two million times, allowing it to see smaller objects and greater detail in these objects. Unlike a light microscope, which uses glass lenses to focus light, the electron microscope uses electrostatic and electromagnetic lens to control the illumination and imaging of the specimen. A schematic diagram of electron microscope is given in fig.

Where d is the smallest distance between two parts of an object, l the wavelength of light used for illumination of the object space and µ sin a is the numerical aperture of the objective, µ being the refractive index of the object space and a is the semi vertical angle of the cone of rays entering the objective. The reciprocal of the limit of resolution gives the resolving power (R. P.). As the wavelength decrease the resolving power increases. Thus using ultra - vilolet light we get greater resolving power than with the ordinary visible light. As shown by De Broglie the wavelength associated with an electron accelerated through V volts is given by

Electrons of energy will have for instance a wavelength of 0.0122 À, a wavelength 50000 times smaller than the wavelength of the visible light. Hence if we use a beam of fast electrons instead of ordinary light we shall obtain much higher resolving power. As such electron microscopes have been constructed and used to reveal still finer details ordinarily impossible to be revealed by optical microscopes.

The two most common kinds of electron microscope are -

(1)  Transmission electron microscope (TEM)

(2)  Scanning electron microscope (SEM)

A transmission electron microscope (TEM) works like a light microscope, transmitting a beam of electrons through a thin specimen and then focusing the electrons to form an image on a screen or on film. This is the most common form of electron microscope (SEM) scans a fine beam of electron onto a specimen and collects the electrons scattered by the surface. This has poorer resolution, by gives excellent 3-dimensional images of surface.