microscope

The scanning electron microscope. Electrons from the electron gun are focused to a fine point on the specimen surface by the lens systems. The beam is moved across the specimen by the scan coils. Secondary electrons are emitted by the specimen surface and pass through the detector, which produces an electrical signal. The signal is passed to an electronic console, and produces an image on a screen.

Terms used to describe an optical microscope. In essence, the optical microscope consists of an eyepiece lens and an objective lens, which are used to produce an enlarged image of a small object by focusing light from a light source. Optical microscopes can achieve magnifications of up to 1,500-2,000. Higher magnifications and resolutions are obtained by electron microscopes.

Instrument for forming magnified images with high resolution for detail. Optical and electron microscopes are the ones chiefly in use; other types include acoustic, scanning tunnelling, and atomic force microscopes.

The optical microscope usually has two sets of glass lenses and an eyepiece. The first true compound microscope was developed in 1609 in the Netherlands by Zacharias Janssen (1580-c. 1638). Fluorescence microscopy makes use of fluorescent dyes to illuminate samples, or to highlight the presence of particular substances within a sample. Various illumination systems are also used to highlight details.

The transmission electron microscope, developed from 1932, passes a beam of electrons, instead of a beam of light, through a specimen. The eyepiece is replaced with a fluorescent screen or photographic plate; far higher magnification and resolution are possible than with the optical microscope.

The scanning electron microscope (SEM), developed in the mid-1960s, moves a fine beam of electrons over the surface of a specimen, the reflected electrons being collected to form the image. The specimen has to be in a vacuum chamber.

The acoustic microscope passes an ultrasonic (ultrahigh-frequency sound) wave through the specimen, the transmitted sound being used to form an image on a computer screen.

In the scanned-probe microscope, developed in the late 1980s, a probe with a tip so fine that it may consist only of a single atom is moved across the surface of the specimen. In the scanning tunnelling microscope, an electric current that flows through a probe is used to construct an image of the specimen. In 1988 a scanning tunnelling microscope was used to photograph a single protein molecule for the first time. In the atomic force microscope, the force felt by a probe is measured and used to form the image. These instruments can magnify a million times and give images of single atoms.