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MICROSCOPES

A microscope is an instrument that magnifies very small objects, allowing the viewer to see detail in the object that is invisible to the naked eye. Microscopes are used mostly, but not only, in biology and medical research.

There are two main types of microscope - optical microscopes and electron microscopes. In an optical microscope, the image of the object is created by light. The simplest optical microscope is a magnifying glass, which contains a single lens. The lens gathers and bends light coming from the object, making the object look larger than it really is. Compound microscopes have more than one lens. A standard compound microscope has two groups of lenses. The first group, called the objective, gathers light from the object and focuses it to create a magnified image of the object. The second group, called the eyepiece, magnifies this image.

The first compound microscope was probably built by Dutch spectacle-maker Zacharias Janssen in about 1590. Early microscopes had poor-quality lenses and gave blurred images. In the 1670s another Dutchman, Anton van Leeuwenhoek, began making simple, single-lens microscopes. He was the first person to see microorganisms, such as bacteria and amoebae.

ELECTRON MICROSCOPES

Optical microscopes can only magnify objects up to 2000 times. Greater magnifications do not reveal any more detail. Electron microscopes can magnify objects more than a million times. In an electron microscope, a beam of tiny particles called electrons does the same job as light in an optical microscope. It is fired at the object and then focused by electromagnetic “lenses” on to a screen that emits light where the electrons hit it.

There are two main types of electron microscope. In a transmitting electron microscope (TEM), the beam of electrons is fired through an extremely thin slice of the specimen under investigation. In a scanning electron microscope (SEM), a very narrow beam of electrons is fired at the surface of the specimen. The beam scans across the surface of the specimen and a sensor detects the electrons bouncing off. In this way, a three-dimensional image of the specimen is gradually built up.

The images created by electron microscopes are called electron photomicrographs. They may be viewed on television screens using video cameras, or digitized and viewed on computer screens.

The first electron microscope, which could magnify objects up to 400 times, was built in 1932 by German engineers Ernst Ruska and Max Knoll. The newest type of electron microscope is the scanning tunneling electron microscope (STM). It can magnify up to 100 million times, which is enough to see individual atoms.

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