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Other Types of Microscopy

Electron Microscopy

Image Source: sv.rkriz.net 
                                        In electron microscope, the source of illumination is electron beam. The construction and principles of electron microscope are easily related to those of light microscope. The range of wavelength of visible light used in light microscope is 4000Å-7000Å, while with an electron microscope employing 60-80KV electron, the wavelength is only 0.05Å. 
In the instrument, the electron gun generates electron beam. These electrons are concentrated by other components of electron gun producing a fast moving narrow beam of electron. Electrons are focused by electromagnetic lenses. Electromagnetic lens consists of wire encased in soft iron casing. When electric current is passed through the coil, it generates an electromagnetic field through which electrons are focused. 
There are three types of electromagnetic lenses. The one is placed between the source of illumination and the specimen. This focuses the beam of electron on specimen functions in a similar manner as that of light microscope. The other two lenses are on the opposite side of specimen which magnify the image in similar fashion as objective and ocular in light microscope. 


TEM (Transmission Electron Microscopy)
Image Source: www.hk-phy.org

The electron source is commonly a tungsten filament of 30-150KV potential. The electron beam passes through a centre of ring like magnetic condenser and becomes converged on the specimen. After being transmitted through the specimen, the magnetic objective focuses the electron into a first image of the object which is enlarged (2000 times). The magnetic projector lens then magnifies a portion of the first image producing magnification up to 240000 or more. 
The final enlarged image can be reviewed by striking a fluorescent screen which makes it visible. The image can also be thrown upon a photographic plate for permanent record. Portions of the photographs may be enlarged four to six times giving the picture in the range of two million times as large as the object. 
Molecules in the microscope interferes with the movement of electrons. To prevent this, interior of the microscope is kept in the state of high vacuum, around 10-4 to 10-6 mm Hg. It is also necessary to have specimen ultra thin. The electron beams has very poor penetrating power, therefore, only small objects or very thin sections of the specimen can be examined. 

(SEM) Scanning Electron Microscopy 

In SEM, electrons are not transmitted through the very thin specimen from below but impinge on its surface from above. The specimen may be opaque and of any manageable thickness and size. if the specimen is an electron conductor, it needs only to be held on an appropriate support. If it is non-conductor, it is allowed to dry but if most, freeze dried in liquid nitrogen is necessary. the specimen is then coated with metal vapour in vacuum. 
The Electrons originate at high energy (20,000 V) from a hot tungsten or lanthanum hexoboride cathode "gun". These electrons are sharply focused, adjusted and narrowed by an arrangement of magnetic fields. Instead of forming a broad inverted cone of rays, in SEM a needle sharp probe is made. This probe acts only as an exciter of image forming secondary electrons emerging from the surface of the specimen. The Probe scans the specimen like that on a blank TV Screen. The probe can impinge on depth and heights with equal speed and accuracy giving great depth of field and producing images with three dimensions. Images are elicited from wherever the probe strikes the metal coated areas of the specimen. Magnification is the ratio of final image to the diameter of area scanned. 
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