ELECTRON MICROSCOPES

An electron microscope uses electrons to “illuminate” an object. Electrons have a much smaller wavelength than light, so they can resolve much smaller structures. The smallest wavelength of visible light is about 4000 angstroms (40 millionths of a meter). The wavelength of electrons used in electron microscopes is usually about half an angstrom (50 trillionths of a meter).

Electron microscopes have an electron gun that emits electrons, which then strike the specimen. Conventional lenses used in optical microscopes to focus visible light do not work with electrons; instead, magnetic fields are used to create “lenses” that direct and focus the electrons. Since electrons are easily scattered by air molecules, the interior of an electron microscope must be sealed at a very high vacuum. Electron microscopes also have systems that record or display the images produced by the electrons.

There are two types of electron microscopes: the transmission electron microscope (TEM), and the scanning electron microscope (SEM). In a TEM, the electron beam is directed onto the object to be magnified. Some of the electrons are absorbed or bounce off the specimen, while others pass through and form a magnified image of the specimen. The sample must be cut very thin to be used in a TEM, usually no more than a few thousand angstroms thick. A photographic plate or fluorescent screen beyond the sample records the magnified image. Transmission electron microscopes can magnify an object up to one million times.

In a scanning electron microscope, a tightly focused electron beam moves over the entire sample to create a magnified image of the surface of the object in much the same way an electron beam scans an image onto the screen of atelevision. Electrons in the tightly focused beam might scatter directly off the sample or cause secondary electrons to be emitted from the surface of the sample. These scattered or secondary electrons are collected and counted by an electronic device. Each scanned point on the sample corresponds to a pixel on a television monitor; the more electrons the counting device detects, the brighter the pixel on the monitor is. As the electron beam scans over the entire sample, a complete image of the sample is displayed on the monitor.

An SEM scans the surface of the sample bit by bit, in contrast to a TEM, which looks at a relatively large area of the sample all at once. Samples scanned by an SEM do not need to be thinly sliced, as do TEM specimens, but they must be dehydrated to prevent the secondary electrons emitted from the specimen from being scattered by water molecules in the sample.

Scanning electron microscopes can magnify objects 100,000 times or more. SEMs are particularly useful because, unlike TEMs and powerful optical microscopes, they can produce detailed three-dimensional images of the surface of objects.

The scanning transmission electron microscope (STEM) combines elements of an SEM and a TEM and can resolve single atoms in a sample.

The electron probe microanalyzer, an electron microscope fitted with an X-ray spectrum analyzer, can examine the high-energy X rays emitted by the sample when it is bombarded with electrons. The identity of different atoms or molecules can be determined from their X-ray emissions, so the electron probe analyzer not only provides a magnified image of the sample, but also information about the sample's chemical composition.

OPTICAL MICROSCOPES

The most widely used microscopes are optical microscopes, which use visiblelight to create a magnified image of an object. The simplest optical microscope is the double-convex lens with a short focal length. Double-convex lenses can magnify an object up to 15 times.

The compound microscope uses two lenses, an objective lens and an ocular lens, mounted at opposite ends of a closed tube, to provide greater magnification than is possible with a single lens. The objective lens is composed of several lens elements that form an enlarged real image of the object being examined. The real image formed by the objective lens lies at the focal point of the ocular lens. Thus, the observer looking through the ocular lens sees an enlarged virtual image of the real image. The total magnification of a compound microscope is determined by the focal lengths of the two lens systems and can be more than 2000 times.

Optical microscopes have a firm stand with a flat stage to hold the material examined and some means for moving the microscope tube toward and away from the specimen to bring it into focus. Ordinarily, specimens are transparent and are mounted on slides—thin, rectangular pieces of clear glass that are placed on the stage for viewing. The stage has a small hole through which light can pass from a light source mounted underneath the stage—either a mirror that reflects natural light or a special electric light that directs light through the specimen.

Microbiology

An agar plate streaked withmicroorganisms

Microbiology (from Greek μῑκρος, mīkros, "small"; βίος, bios, "life"; and -λογία, -logia) is the study ofmicroorganisms, which are unicellular or cell-cluster microscopic organisms. This includes eukaryote such as fungi and protists, and prokaryotes, which are bacteria and archaea. Viruses, though not strictly classed as living organisms, are also studied. In short; microbiology refers to the study of life and organisms that are too small to be seen with the naked eye.

Microbiology is a broad term which includes virology, mycology, parasitology, bacteriology and other branches. A microbiologist is a specialist in microbiology.

Microbiology is researched actively, and the field is advancing continually. We have probably only studied about one percent of all of the microbe species on Earth. Although microbes were first observed over three hundred years ago, the field of microbiology can be said to be in its infancy relative to older biological disciplines such as zoology and botany.