Magnification and resolution

The magnification of an instrument is the increase in the apparent size of the object. The total magnification of a compound microscope is worked out by multiplying the magnification of the objective lens by that of the ocular lens.

There is virtually no limit to the magnification produced by a light microscope; it depends on the power of the lenses used. However, above a certain magnification the image becomes blurred and it is impossible to distinguish structures lying close together. This limit of effective magnification is called the resolving power or resolution of the microscope. It is defined as the ability of a microscope to show two objects as separate. The resolving power of the light microscope is limited by the wavelength of light. Light microscopes can magnify objects up to about 1500 times without losing clarity.

The electron microscope

Electron microscopes use a beam of electrons instead of a beam of light. Electron beams have a much smaller wavelength than light rays, so electron microscopes have greater resolving powers and can produce much higher effective magnifications than light microscopes. There are two main types of electron microscopes: the transmission electron microscope (TEM), and the scanning electron microscope (SEM).

The transmission electron microscope

T

he TEM is used to study the details of the internal structure of cells. Extremely thin samples of the specimen are needed. To make these the specimen is supported in a resin block to prevent in collapsing during cutting, and is sliced with a diamond or glass knife. The section is then impregnated with a heavy-metal stain, such as osmium tetroxide.

As the beam passes through the specimen, electrons are absorbed by heavily stained parts but pass readily through the lightly stained parts. Electromagnets bend the electron beam to focus an image onto a fluorescent screen or photographic film. Photograph taken through an electron microscope is called an electron micrograph.

The most modern TEMs distinguish objects as small as 0.2nm. This means that they can produce clear images magnified up to 250 000 times. The magnification is varied by changing the strength of the electromagnets.

The scanning electron microscope

The SEM is used to produce three-dimensional images of the surface of specimens. Electron are reflected from the surface of a specimen stained with a heavy metal. This enables the SEM to produce images of whole specimens: cells, tissues, or even organisms.

A

lthough electron microscopes have revolutionized cell biology, they have not completely replaced light microscopes. Light microscopes are used to examine living and unstained specimens. Preparation of specimens for electron microscopy is complicated and time-consuming. Electron microscopes are very expensive and can be used only to study dead specimens stained with heavy metal, which might well produce artifacts.

■ Glossary of essential terms for you to know.

№	English term	Russian equivalent
1	magnified	увеличенный
2	specimen	образец, особь
3	to explore	исследовать
4	to invent	изобретать
5	lens	линза
6	to refract	преломлять
7	magnification	увеличение
8	apparent	видимый
9	compound	составной, сложный
10	blurred	смазанный
11	to distinguish	различать
длина волны12	resolution	разрешение
14wavelength 13	clarity	ясность, четкость
15	beam	луч, пучок лучей
16	ray	луч
17	internal	внутренний
18	sample	образец
19	to impregnate	оплодотворять, пропитывать, насыщать
20	stain	пропитка
21	readily	легко
22	to reflect	отражать
23 to enable	позволять
24	tissues	ткани
25	to replace	заменять

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