Microscope
A microscope magnifies and resolves the image of an object that otherwise would be invisible to the naked eye. These objects include such items as human skin, the eye of a fly, cells of a living organism, viruses, individual molecules, and atoms.
The first optical microscopes, those that produce images through the use of visible light, used drops of water captured in a small hole to function as a magnifying lens. Later, magnifying glasses consisting of a single biconvex lens were developed and were capable of enlarging an object up to 20 times its original size as seen by the naked eye. (Scientists describe the power of magnification by writing a number followed by a times sign (x), so this would be expressed as 20x.)
Using several lenses in conjunction with each other is the secret of a microscope's success, however. Dutch spectacle maker, Zacharias Janssen, devised the first compound microscope in 1590. Today's basic optical compound microscopes consist of a two-lens system with an objective and an ocular lens, and can magnify up to 2,000x. The specimen sits close to the objective lens, which magnifies the images as an ordinary magnifying glass would. The lens forms an enlarged real image of the specimen near the ocular or eyepiece lens. The eyepiece works in conjunction with the objectives to correct for aberration and further magnify the image, creating the virtual image seen in a microscope. Light reflected from a mirror and concentrated by the condenser lenses located under the specimen stage, illuminates the object.
Various types of optical microscopes
The familiar monocular compound optical microscopes are being replaced in many laboratories with binocular styles. These microscopes have a single objective lens, but two ocular ones, each in its own eyepiece. Light coming through the objective lens is split into two beams by a prism. Each eye sees the exact same image, so there is no three dimensional effect.
For a three-dimensional view, scientists use a stereoscopic binocular microscope. This instrument consists of two separate sets of objective lenses as well as two separate ocular lenses. Prisms alter the angle of light coming through each pair of lenses, so each eye sees a slightly different image.
Living or stained specimens often yield poor images when viewed in bright-field illumination. To help with this, scientists developed a phase-contrast microscope that alters the phase differences in light waves as they pass through the specimen. This makes some parts of the object brighter and others darker than normal, allowing for a better view of the structural details of the object. Closely related to this type is the interference microscope that superimposes one field of view over a second to improve contrast.
Particularly useful for biological studies, a dark-field microscope uses a specialized illumination technique that capitalizes on indirect illumination to enhance contrast in specimens. The stop, an opaque disk set in a condenser under the stage, prevents illuminating light from shining directly on the specimen. Instead, illuminating light passes around the stop and is reflected off the condenser's walls.
To observe color in cells, scientists use polarizing microscopes. The microscope aligns the vibrations of a light wave by directing it through a specially cut prism. If two beams of polarized light are transmitted through the cell, as in a differential polarization microscope, researchers can make quantitative measurements.
Electron microscope
Prior to 1930, all microscopes were optical. In 1931, German physicist Ernst Ruska developed the electron microscope that used a beam of moving electronics to illuminate an object instead of light. Magnetic lenses or electric coils produce magnetic fields to deflect the electrons in the same manner that glass lenses bend light rays. The specimen has to be in a vacuum, however, because electrons cannot travel through air. Electron microscopes give point-to-point resolutions of less than 0.2 nanometers. This high resolution permits the direct visualization of many molecules and some atoms.
The transmission electron microscope (TEM) images specimens a fraction of a micrometer or less in thickness. In a TEM, the beam passes through the specimen so that some of the electrons are absorbed and some scattered. The remaining electrons are focused onto a fluorescent screen or special photographic plates via the use of magnetic lenses. The resulting image is in black and white.
In the scanning electron microscope (SEM), a narrowly focused electron beam is scanned over the surface of a solid object and used to build up an image of the details of the surface structure through reflection. Researchers use this type to study minute details on a surface of an object. These microscopes created 3D images that are magnified up to 50,000x.
Although there are several other special types of electron microscope, perhaps the most valuable is the electron-probe microanalyzer, which allows a researcher to make a chemical analysis of the composition of materials. This type of microscope uses the incident electron beam to excite the emission of characteristic x radiation by the various elements composing the specimen. Spectrometers built into the instrument detect and analyze the x rays. Viewing the resulting image, the researcher can easily correlate the structure and composition of the material.
Other types of microscopes
Scanning tunneling microscopes do not look like conventional microscopes at all. These are used to resolve individual atoms, identifying details down to one-tenth of an angstrom in height and less than two angstroms in width. Instead of lenses or mirrors, this microscope sports a tungsten rod with a tip is made up of a pyramid of atoms and three pieces of piezoelectric crystal, which compress and stretch in response to changes in the voltage of an electric charge. Electrons "tunnel" or flow through the vacuum or water from the tungsten tip to the atoms on an object's surface, creating a current that reacts with the crystal. Its inventors, Gerd Binnig of West Germany, and Heinrich Rohrer of Switzerland, won the Nobel Prize in 1986 for this development. They shared the prize with Ernst Ruska.
In 1986, the atomic force microscope (AFM) debuted. The AFM produces three dimensional images of surfaces both in air and under liquids at a resolution of nanometers, or billionths of a meter. In its contact mode, the AFM lightly touches a tip at the end of a 50-300 micrometer long leaf spring (the cantilever) to the sample. As the tip is scanned over the sample, a detector measures the vertical deflection of the cantilever, yielding the precise height of the sample at local points. The deflections of the cantilever are monitored by a laser beam that is reflected off the cantilever and into a position-sensitive detector. Force sensitivities and position accuracy as small as 10-15 Newtons/Hz1/2 and 0.01 nanometer, respectively, can be measured using microfabricated cantilevers. If the tip and sample are coated with two types of molecules, an AFM can measure force of attraction or repulsion between them, potentially at the level of a single hydrogen bond. Since its invention, the atomic force microscope has permitted high-resolution imaging at the subnanometer level. More recently, scientists introduced the microscope to a liquid environment and the resolution improved to the atomic level.
This is the complete article, containing 1,166 words
(approx. 4 pages at 300 words per page).
