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Types of Microscope: Optical and Electron Microscope (With Figure) Microorganisms are usually not visible to naked human eye. They can be made visible, only when they are magnified under microscopes. Microscopes are instruments, which can produce enlarged images of very small objects, making it possible to view them, which, otherwise, cannot be seen distinctly by naked human eye. Different types of microscopes are used by the microbiologists for specific purposes. Microscopes are of the following types (Figure 4.1). A. Light or Optical Microscopes: In light or optical microscopes, ‘light waves’ are used to produce the enlarged images of very small objects and magnification is obtained by a system of ‘optical lenses’. Ordinarily, microbes do not absorb much light, but staining them with dyes greatly increases their light absorbing ability, resulting in greater contrast and color differentiation. B. Light microscopes are of four types as described below: (1) Bright-field Microscope: In a bright-field microscope, the microscopic field (the circular area visible under microscope) is brightly illuminated and the microbes (or biological specimen) appear darker, as they absorb some of the light passing through them. It is of two types as follows: (a) Simple Microscope: A simple microscope is used to obtain small magnifications. A single biconvex lens magnifies the size of the object to get an enlarged virtual image. Page 1 of 13 (b) Compound Microscope: The most commonly used microscope for general purposes is the standard compound microscope. It magnifies the size of the object by a complex system of lens arrangement. It has a series of two lenses; the objective lens and the ocular lens, to magnify the size of the object. 2. Dark-field Microscope: In a dark-field microscope, the object is brilliantly illuminated against a dark background. 3. Fluorescence Microscope: It is used to view fluorescent microbes and microbes stained with fluorescent dyes for specific purposes. 4. Phase-contrast Microscope: With a phase-contrast microscope, the differences among various cells with different refractive indices or thickness can be seen in unstained condition. Unstained structures within cells, not discernible by most other microscopes can also be observed, due to the slight differences in their refractive indices or thickness. It is a compound microscope fitted with a phase-contrast condenser and a phase-contrast objective. B. Electron Microscopes: In electron microscopes, ‘electron beams’ are used to produce the image of the object and magnification is obtained by a system of ‘electromagnetic fields’, unlike in light microscopes, in which Tight waves’ are used to produce the image of the object and magnification is obtained by a system of ‘optical lenses’. The resolving power of electron microscopes is 200 times greater than that of light microscopes. They can produce useful magnifications up to X 400,000, as compared to X 2000 in light microscopes. Thus, the useful magnification is 200 times greater in electron microscopes than in light microscopes. Electron microscopes are of three types as described below: 1. Transmission Electron Microscope (TEM): In this microscope, electron beam is transmitted through an ultra-thin section of the 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): This microscope uses an electron beam to scan the surface of the object, thereby inducing it to release a shower of electrons, which are collected by a detector to generate the image. It is used to observe the surface structure of microscopic objects. Page 2 of 13 3. Scanning and Transmission Electron Microscope (STEM): 3. Scanning and Transmission Electron Microscope (STEM): It has both transmission and scanning electron microscope functions. The Different Types of Microscopes The invention of the microscope has brought about great inventions that have transformed the human race. This device is effective and very important in science because it provides opportunity for scientist to study natural elements that are not visible to the naked eye. There are different types of microscopes depending on the purpose for which it is intended. Microscopes can also be classified based on their image making physical principles, area of application and versatility. However, microscopes can be comfortable divided into two categories, light microscope and electron microscope. Below is a list of the major microscope types and their uses. Light or Compound Microscopes These types of microscopes are based on a simple principle of light and lens. A light source illuminates the object while the lens magnifies it so that it can be visible to the human eye for the purpose of studying or evaluating. Under this category are the simple microscope, compound microscope and the stereo microscope. Page 3 of 13 Simple Microscope This is one of the oldest microscopes that uses a single lens for magnifying any samples. However, this microscope has been regarded as primitive because of its less relevance in serious scientific work. Compound Microscopes This type of microscope operates on the same principle as the as the simple microscope. But the difference is that it makes use of two different optical parts for the magnifying of objects. The compound microscopes are the most commonly used in many laboratories because they are efficient, inexpensive, and can magnify objects as much as 2000 times the original size. They are used mainly for the study of cells, chromosomes and the DNA. Dissection or Stereo Microscope This is another member of the optical microscopes that makes use of light and lens. This microscope is different from other types of microscopes because it allows you to view objects in 3D. It contains lens in different angles that provides a three dimensional viewing of objects for complete diagnosis. However, the stereo microscope doesn’t have very strong magnifying power like the compound microscope, but can be very useful in studying of dissection parts of living organisms. It is used mainly in the field of medical science including forensics, fine repair, sorting, and microsurgery. Other types of optical microscopes that are not very common but still well used includes the UV microscope that makes use of UV light to observe objects, the inverted microscope that is used for viewing thick or large objects upside down, and the metallurgic microscope used by engineers and scientists for viewing the structure of metals, ceramic and plastic. There are also digital microscopes that make use of optical lens and CCD/CMOS sensors to magnify objects to about 1000 times. Digital microscopes are also good because they have a 2 million pixel camera that provides high quality recording of the objects in view, and is connected to a TV monitor for high resolution viewing or observation. Electron Microscopes These are the most advanced types of microscopes used in modern science. The electron microscopes are powered by a beam of electron that strikes any objects that comes to its path to magnify it. Electron microscopes are used for studying cells and small particles of matter, as wells as large objects. Types of electron microscopes include transmission, scanning and reflection electron microscopes. Page 4 of 13 Transmission Microscope Transmission microscopes are used for studying cells and tiny slices of microorganisms like viruses, after they have been stained with palladium and gold and placed upon a wire grid. Scanning Electron Microscope Scanning electron microscopes have lower magnifying power but can provide 3 dimensional viewing of objects. The image of the object is captured in black and white after being stained with gold and palladium. Reflection Electron Microscope Reflection electron microscopes also uses electron beams but is different from transmission and scanning electron microscopes being that it is build to detect electrons that have been scattered elastically. All types of microscopes are used based on their purpose and the results that the scientist or the observer is trying to achieve. There are other microscopes designed for specific use in different types of field or based on their source like the X-ray microscope that uses X-ray beams to create images of an object. And the scanning acoustic microscope that makes use of sound waves to detect images. This type of microscope is used material science and biological science for detecting cracks in material and to uncover elasticity and stress in biological structures Page 5 of 13 respectively. History of (includes: Who invented the microscope) the Microscope During the 1st century AD (year 100), glass had been invented and the Romans were looking through the glass and testing it. They experimented with different shapes of clear glass and one of their samples was thick in the middle and thin on the edges. They discovered that if you held one of these “lenses” over an object, the object would look larger. Someone also discovered that you can focus the rays of the sun with one of these special “glasses” and start a fire. These early lenses were called magnifiers or burning glasses. The word lens by the way, is derived from the latin word lentil, as they were named because they resembled the shape of a lentil bean (look up lens in a dictionary). These lenses were not used much until the end of the 13th century when spectacle makers were producing lenses to be worn as glasses. The early simple “microscopes” which were really only magnifying glasses had one power, usually about 6X - 10X . One thing that was very common and interesting to look at was fleas and other tiny insects. These early magnifiers were hence called “flea Page 6 of 13 glasses”. Sometime about the year 1590, two Dutch spectacle makers, Zaccharias Janssen and his father Hans started experimenting with these lenses. They put several lenses in a tube and made a very important discovery. The object near the end of the tube appeared to be greatly enlarged, much larger than any simple magnifying glass could achieve by itself! They had just invented the compound microscope (which is a microscope that uses two or more lenses). Galileo heard of their experiments and started experimenting on his own. He described the principles of lenses and light rays and improved both the microscope and telescope. He added a focusing device to his microscope and of course went on to explore the heavens with his telescopes. Anthony Leeuwenhoek of Holland became very interested in lenses while working with magnifying glasses in a dry goods store. He used the magnifying glass to count threads in woven cloth. He became so interested that he learned how to make lenses. By grinding and polishing, he was able to make small lenses with great curvatures. These rounder lenses produced greater magnification, and his microscopes were able to magnify up to 270X! Anthony Leeuwenhoek became more involved in science and with his new improved microscope was able to see things that no man had ever seen before. He saw bacteria, yeast, blood cells and many tiny animals swimming about in a drop of water. From his great contributions, many discoveries and research papers, Anthony Leeuwenhoek (1632-1723) has since been called the "Father of Microscopy". Robert Hooke, an Englishman (who is sometimes called the “English Father of Microscopy”), also spent much of his life working with microscopes and improved their design and capabilities. Little was done to improve the microscope until the middle of the 19th century when great strides were made and quality instruments like today’s microscope emerged. Companies in Germany like Zeiss and an American company founded by Charles Spencer began producing fine optical instruments. Today, there are no microscope manufacturers in the US and most of the microscopes come from Germany, Japan and China. Toy plastic microscopes should be avoided as they do not achieved the level of quality of the basic instruments with metal frames and glass lenses. Because of foreign production, quality microscopes have become affordable for all. Zaccharias Janssen, the inventor of the microscope would marvel at the quality of even Page 7 of 13 the most basic microscopes found in schools today. Page 8 of 13 Page 9 of 13 Page 10 of 13 Page 11 of 13 Page 12 of 13 Page 13 of 13