The electron microscope, like optical microscope, is an instrument principally used in the research laboratory for magnifying small objects. The wave nature of moving electron is the basis for the electron microscope. The resolving power of a microscope is the least distance between two points which can be distinguished. The resolving power of a microscope is limited by the wave length of the radiation used. In optical microscope, the visible light is used to illuminate the object and the highest magnification obtained with the best optical microscope is about 2000. Since, the wavelength of X-rays is smaller than that of the visible light, one can think of having an X-ray microscope. However, X-rays cannot be focussed as visible radiations are focussed using lenses. On the other hand, electrons having de Broglie wavelength of the order of X-rays can be focussed easily using electric and magnetic fields and one can build a high resolving power microscope using electrons.
For electrons accelerated by a potential difference of about 60,000 volts, the wavelength is about 5 × 10- 12 m. This is 105 times smaller than that of visible light. Hence the resolving power of an electron microscope will be 1,00,000 times greater than that of an optical microscope.
Construction and working
The schematic diagram of an electron microscope is shown in Fig. An electron microscope is similar in principle to an optical microscope. The modern electron microscope is usually of transmission type in which magnetic lenses of short focal length are used to obtain large magnification. An electron beam emitted by a filament is accelerated through a large potential difference in a device called electron gun. The fine beam of electrons is made to pass through the centre of the doughnut shaped electromagnet A (condenser magnetic lens). The electrons get deflected to form a parallel beam which strikes the object to be magnified. It should be noted that the electrons will be transmitted more through the transparent parts of the object and transmitted in less number through comparatively denser portions. The transmitted beam will thus have the likeness of the object traversed by it. The second electro magnet B (objective magnetic lens) causes the electron beam to diverge to produce enlarged image of the object. The electromagnet C (projector magnetic lens) focusses the electron beam from the part of the enlarged image on the fluorescent screen producing still greater magnification. The image obtained on the fluorescent screen is made visible by scintillation for direct view. It can also be obtained on a suitable photographic plate for a permanent record. Sharp focussing is obtained by adjusting the intensity of magnetic fields produced by electro magnets. Since, the electron beam operates in vacuum, the apparatus is mounted in a chamber which is completely evacuated.
1. It is used in the industry, to study the structure of textile fibres, surface of metals, composition of paints etc.
2. In medicine and biology, it is used to study virus, and
4. In Physics, it has been used in the investigation of atomic structure and structure of crystals in detail.
An electron microscope is operated only in high vacuum. This prohibits the use of the microscope to study living organisms which would evaporate and disintegrate under such conditions.