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The transmission electron microscope
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.
■ Glossary of essential terms for you to know.
■ Your Essential Assignments I. Quick check 1. How is the magnification varied in: a) a light microscope b) an electron microscope? 2. Why is the resolving power of an electron microscope so much better than of a light microscope? 3. What is the approximate size of the smallest structure that can be observed with a light microscope?
II. Fill in the missing words:
III. Use monolingual English dictionary and write down what could the words given below mean: microscope, to refract, magnification, sample, ray.
IV. Match these words with their definitions:
V. Find English equivalents to the following word combinations:
VI. Give Russian equivalents to the following English terms:
VII. Find synonyms among the pool of words:
VIII. Answer the following questions. Use all information given before: 1. What are microscopes used for? 2. What types of microscopes are most commonly used today? 3. What is a compound light microscope? 4. What does the magnificationof an instrument depend on? 5. How do electron microscopes differ from compound light microscopes? 6. What are the main types of electron microscopes? 7. What is the difference between thetransmission electron microscope and the scanning electron microscope?
IX. Match the sentence halves. Make complete centences: 
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The 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.
Although 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.