Introduction to Microscope
Microscope, an instrument used to obtain enlarged images of tiny objects or minute details of objects. Microscopes are widely used in the physical and natural sciences. In medicine, the microscope makes possible the identification of many of the bacteria and other microorganisms that cause disease. In physics, powerful microscopes reveal the molecular structure of matter. Cameras can be used with most types of microscopes to provide a photographic record of observations.
The simplest microscope is an ordinary magnifying glass—a single double-convex lens. Magnifying glasses are commercially available that will make an object appear as much as 10 times its actual size. Such a glass is said to magnify 10 diameters, or 10 X. A compound microscope—one having two or more lenses—will give much greater magnification. Most powerful of all are electron microscopes and field ion microscopes.
Kinds of Microscopes
Optical MicroscopesIn an optical microscope, the image is formed with visible light. (With a converter, ultraviolet light may also be used.) Most laboratory microscopes are compound optical microscopes. The lower system of lenses is called the objective and the upper system, made up of ocular lenses, is the eyepiece, or ocular. There is usually a set of three or four objectives, mounted on a revolving turret, to provide varying degrees of magnification. A binocular microscope has twin eyepieces, making it more comfortable to use for long periods than an ordinary compound microscope. A stereoscopic microscope, with twin objectives as well as twin eyepieces, gives a three-dimensional image.
The specimen to be studied is placed on a piece of glass, called a slide. If the specimen is translucent—a drop of water, for example —light may be directed through it. If the specimen is opaque—a piece of metal, for example—the light must be reflected from its surface. The light may come from an outside source such as a window, or from a lamp attached to the microscope. A mirror controls the angle at which the light strikes the specimen. In most high-quality microscopes, a lens called a condenser is placed between the mirror and the specimen to insure even illumination.
When the lenses are properly focused, the objective forms an enlarged, inverted image, called a real image, inside the focus of the eyepiece. The eyepiece magnifies this image further and makes a larger virtual image visible to the observer. The total magnification of an optical microscope is obtained by multiplying the magnifying power of the objective by that of the eyepiece. For example, a microscope that has an eyepiece with a magnification of 10 and an objective with a magnification of 10 has a total magnification of 100 diameters. Magnification with an optical microscope is generally limited to a few thousand diameters. A television attachment can be used to increase the contrast of the image obtained from an optical microscope and to make the magnification several times greater. The television pictures can be recorded on videotape.
Electron MicroscopesIn an electron microscope, electrons, rather than light, form the image. Any air molecules in the interior of the microscope would distort the path of the electrons; thus, air must be evacuated from the microscope for its operation. Since electrons are not visible, the image they form is viewed either by projecting them onto a fluorescent screen, where they form a visible image, or by using them to produce signals for creating an image on a television screen.
In a transmission electron microscope, an electron gun—a device that produces a stream of electrons—replaces the light source of the optical instrument. Electromagnets, instead of transparent discs, are used as lenses. An extremely thin section of the specimen is mounted inside the microscope and the stream of electrons is directed at it. After passing through the section of specimen, the electrons are focused into an image by the electromagnets. A transmission electron microscope can magnify to about 200,000 diameters. A video attachment can be used to intensify the image and increase total magnification 10 times or more.
In a scanning electron microscope, or SEM, a stream of electrons is focused into a narrow beam by electromagnets and direct-ed at a full specimen, rather than a thin section of one. The beam is made to scan, or sweep over, the specimen in a linear pattern. As the beam reaches a given point on the specimen, the electrons in the beam strike electrons of the atoms at the surface and eject some of the electrons from the specimen. When the ejected electrons reach a detecting device facing the specimen, they produce a signal that is used to create a magnified image on a television screen.
The highlights and shadows in an SEM image provide a three-dimensional effect, with virtually all of the specimen in sharp focus. (Optical microscopes and other electron microscopes render only a thin portion of a specimen in focus at one time.) The SEM can magnify up to about 100,000 diameters; it also can be adjusted down to a magnification of as little as 5 diameters, useful when viewing relatively large objects such as miniaturized electronic devices and small fossils.
A scanning transmission electron microscope combines features of both the scanning and the transmission type. It uses a narrow beam of electrons to scan the specimen; the electrons transmitted through the specimen are used to form the image.
Acoustic MicroscopesAn acoustic microscope uses sound waves to form an image. In a typical acoustic microscope, sound waves of a very high frequency are reflected off the specimen and converted into electrical signals by a device similar to a microphone. The signals are used to produce an image that is enlarged electronically and displayed on a television screen. Slight differences in the elastic properties of the materials that make up the various parts of the specimen affect the reflection of the sound waves and, therefore, the strength of the electrical signals and the pattern they produce on the screen.
The magnification of an acoustic microscope can be as great as that of the best optical microscopes. Acoustic microscopes are especially suited for studying the microscopic structure and composition of metals and alloys, for inspecting microelectronic circuits, and for observing living cells without having to apply stains to make transparent structures visible.
X-Ray MicroscopesSeveral types of microscopes use X rays to form an image. Some use techniques involving diffraction to focus the X rays. (X rays, unlike light waves, are not reflected or refracted.) In a type of X-ray microscopy called contact (or flash) X-ray microscopy, a short burst of X rays is directed toward the specimen. The X rays that pass through the specimen form an image on a sensitized plate held against it. When the plate is developed, areas of the plate's surface are etched in proportion to their exposure, producing a three-dimensional image that is studied with a scanning electron microscope.
Ion MicroscopesSome microscopes use positively charged atoms, called ions, to produce an image. The most common type is the field ion microscope, which uses ions from the specimen itself to produce an image. It is used mainly to study the atomic or molecular structure of metals. The metal to be studied is usually placed at the tip of a fine needle. In the microscope, a high voltage causes ions to fly off from the metal at the tip and hit a detecting screen. The ions fan out as they travel, so that the image of the tip is magnified up to 5,000,000,000 diameters.
Scanning-probe MicroscopesA variety of microscopes produce an image by means of a minute needlelike probe that is moved back and forth across the specimen. The most important is the scanning tunneling microscope. In this microscope, the probe is held at an extremely short distance—about the width of an atom—above the surface of the specimen. At this distance, electrons pass between the specimen's surface and the probe by a phenomenon called tunneling. In the most common version of the microscope, a voltage is applied to create a current between the specimen and the probe, and a feedback mechanism automatically regulates the height of the scanning probe to keep the current constant. The vertical movement of the probe is used to plot the relief of the surface. Such a plot is able to show the individual atoms that make up the surface of the specimen.
Other types of scanning-probe microscopes use probes that are sensitive to the electrostatic charge, thermal activity, or certain other properties of the atoms and molecules that make up the surface of the specimen.
History of the Microscope
The compound optical microscope was invented in the Netherlands early in the 1600's. Some historians have credited Zach-arias Janssen with the invention. Because of difficulties in manufacturing reliable lenses, the instrument was not widely used until the 19th century. In the 17th century, Anton van Leeuwenhoek, a Dutch biologist, constructed and used excellent single-lens microscopes. Through these instruments, he discovered bacteria and protozoans.
The concept of the transmission electron microscope was developed in Germany during the 1920's and 1930's. Ernst Ruska, a German scientist, shared the 1986 Nobel Prize in physics for his work on the instrument. The scanning electron microscope was developed in the United States in the 1960's; the acoustic microscope, in the 1970's. In the late 1970's, Gerd Binnig, a West German scientist, and Heinrich Rohrer, a Swiss scientist, developed the scanning tunneling microscope, for which they shared with Ruska the 1986 Nobel Prize in physics.