Electron Microscope Explained with MCQs

History of Electron microscope

In 1931, physicist Ernst Ruska and electrical engineer Max Knoll first invented the electron microscope. This was the practical application of electron microscopy.

Principle

The electron microscope works on the wave behavior of electrons. The beam of electrons is used to magnify and resolve the image by its characteristic wave nature and provide information on the composition, morphology, structure, and other details of the specimen.

Construction

Electron microscope consists of an electron gun, magnetic condenser, magnetic objective, intermediate image projector, and a fluorescent screen. All parts are described below in detail.

Electron gun

The electron gun in the microscope generates the beam of electrons with uniform velocity. Generally, a potential difference of 30 kV to several megavolts is used to highly accelerate the electrons.

Commercially used high voltage microscopes can operate on potential differences up to 1500 kV.

Magnetic condenser

There are two sets of magnetic condensers which focus the electron beam on the specimen. In this, electric and magnetic fields are used for focusing beams of electrons.

Magnetic objective

It is like the objective lens of an optical microscope. Electrons fall on the specimen scattered out from the thicker part and enter the magnetic objective so the first image of the specimen is produced.

Intermediate image projector

This is like the projector lens of an optical microscope. It is a magnetic coil that forms a real intermediate image and the projector lens shows this on a fluorescent screen.

Fluorescent screen

The highly magnified and resolved image of the object or specimen under study is shown on a fluorescent screen.

The special film on which the final image of the specimen is displayed is called an electron micrograph.

Working of Electron Microscope

Stream of electrons are produced and sped up by the electron gun. The electron beam is made to travel through the center of the magnetic condensing lens. These electrons are made as parallel beam and are focused on the objects.

The electrons are sent more in the less thick region of the object and are transmitted less (i.e.,) absorbed up by the denser region of the object.

Therefore, the transmitted electron beam falling over the magnetic objective lens forms a magnified real image. Even more, the image can be amplified by the magnetic projector lens and the final image is obtained on the fluorescent screen.

In order to make a permanent record of the image of the things, the final image can also be obtained on a photographic plate.

Types of Electron Microscope

Today there are two significant kinds of electron microscopic lens used in clinical and biomedical research: the transmission electron microscope (TEM) and the scanning electron microscope (SEM); in some cases, the TEM and SEM are combined in one instrument, the scanning transmission electron microscope (STEM):

TEM: amplifies 50 to ~ 50 million times; the specimen appears flat.

SEM: amplifies 5 to ~ 500,000 times; sharp images of surface area features.

STEM: amplifies 5 to ~ 50 million times; the specimen appears flat.

Scanning Electron microscope

Scanning electron microscope is an enhanced design of an electron microscopic lens. SEM is utilized to study the 3-dimensional image of the specimen.

When the accelerated primary electrons strike the specimen, it produces secondary electrons. These secondary electrons are collected by a positiv charged electron detector which in turn provides a 3- dimensional image of the specimen.

Transmission electron microscope

In the TEM, the electrons from the electron gun travel through a condenser lens prior to encountering the specimen, close to the objective lens. The majority of the magnification is accomplished by the objective lens system.

The image is viewed at the base of the column and photographed utilizing film, or more recently a CCD camera, by raising the hinged fluorescent viewing screen.

Applications and Advantages

• Electron microscopes are utilized to investigate the ultrastructure of a wide variety of biological and inorganic specimens consisting of microbes, cells, macromolecules, biopsy samples, metals, and crystals.
• Industrially, electron microscopes are frequently used for quality control and failure analysis.
• The science of microbiology owes its advancement to the electron microscope. Study of microbes like germs, virus and other pathogens have made the treatment of illness really reliable.

Advantages

• It can produce magnification as high as 1, 00,000 times as that of the size of the object.
• The focal length of the microscopic system can differ.

MCQs about Electron Microscope