Question

Explain the working mechanism of electron microscopy (including SEM and TEM).

Answer 1

An electron microscope uses an ‘electron beam’ to produce the image of the object and magnification is obtained by ‘electromagnetic fields’; unlike light or optical microscopes, in which ‘light waves’ are used to produce the image and magnification is obtained by a system of ‘optical lenses’.

That is why, despite its smaller numerical aperture, an electron microscope can resolve objects as small as 0.001µ (=10 Å), as compared to 0.2µ by a light microscope. Thus, the resolving power of an electron microscope is 200 times greater than that of a light microscope. It produces useful magnification up to X 400,000, as compared to X 2000 in a light microscope. Thus, the useful magnification is 200 times greater in an electron microscope than that in a light microscope.

1) Transmission Electron Microscope (TEM):

In this microscope, an electron beam from an electron gun is transmitted through an ultra-thin section of the microscopic object and the image is magnified by the electromagnetic fields. It is used to observe finer details of internal structures of microscopic objects like bacteria and other cells.

(2) Scanning Electron Microscope (SEM):

In a scanning electron microscope, the specimen is exposed to a narrow electron beam from an electron gun, which rapidly moves over or scans the surface of the specimen . This causes the release of a shower of secondary electrons and other types of radiations from the specimen surface.

The intensity of these secondary electrons depends upon the shape and the chemical composition of the irradiated object. These electrons are collected by a detector, which generates electronic signals. These signals are scanned in the manner of a television system to produce an image on a cathode ray tube (CRT).

The image is recorded by capturing it from the CRT. Modern variants have facility to record the photograph by digital camera. This microscope is used to observe the surface structure of microscopic objects.