Magnification and resolution
The magnificationof 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 poweror 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: thetransmission electron microscope(TEM), and the scanning electron microscope(SEM).
The transmission electron microscope
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.
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.
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.
■ Glossary of essential terms for you to know.
| № | English term | Russian equivalent |
| magnified | увеличенный | |
| specimen | образец, особь | |
| to explore | исследовать | |
| toinvent | изобретать | |
| lens | линза | |
| to refract | преломлять | |
| magnification | увеличение | |
| apparent | видимый | |
| compound | составной, сложный | |
| blurred | смазанный | |
| to distinguish | различать | |
| resolution | разрешение | |
| wavelength | длина волны | |
| clarity | ясность, четкость | |
| beam | луч, пучок лучей | |
| ray | луч | |
| internal | внутренний | |
| sample | образец | |
| to impregnate | оплодотворять, пропитывать, насыщать | |
| stain | пропитка | |
| readily | легко | |
| to reflect | отражать | |
| to enable | позволять | |
| tissues | ткани | |
| to replace | заменять |
■ 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:
| Term (verb) | Noun | Adjective |
| magnify | ....... | ....... |
| multiply | ....... | ....... |
| reflect | ....... | ....... |
| absorb | ....... | ....... |
| prevent | ....... | ....... |
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:
| 1. | beam | A. | a piece of curved glass which makes things look bigger or smaller |
| 2. | toinvent | B. | the power of a microscope to give a clear picture of things, or a measure of this |
| 3. | eyepiece | C. | to change the position of the lens on a microscope so that you can see something clearly |
| 4. | lens | D. | to make a substance spread completely through something |
| 5. | resolution | E. | a shining line of light from the sun, a lamp |
| 6. | to focus | F. | a picture of a subject in a mirror or in the lens of a camera |
| 7. | specimen | G. | easily noticed |
| 8. | to impregnate | H. | to make, design, or produce something new for the first time |
| 9. | image | I. | made up of two or more parts or substances |
| 10. | apparent | J. | a small amount or piece of something that is taken from a plant or animal, so that can be tested or examined |
| 11. | compound | K. | the glass piece that you look through in a microscope |
V. Find English equivalents to the following word combinations:
| № | Russian term | English equivalent |
| 1. | научное использование микроскопов | |
| 2. | электронный микроскоп | |
| 3. | стеклянные линзы | |
| 4. | окуляр | |
| 5. | видимый размер объекта | |
| 6. | зависеть от ч.-л. | |
| 7. | длина волны света | |
| 8. | без потери четкости | |
| 9. | сканирующий электронный микроскоп | |
| 10. | внутренняя структура клетки | |
| 11. | легко проходить через ч.-л. | |
| 12. | трехмерное изображение | |
| 13. | поверхность образца |
VI. Give Russian equivalents to the following English terms:
| № | English term | Russian equivalent |
| a magnified image of sth. | ||
| a compound light microscope | ||
| to pass through | ||
| to refract (bend) the light | ||
| structures lying close together | ||
| the resolving power | ||
| to be limited by sth. | ||
| a beam of light | ||
| thetransmission electron microscope | ||
| to focus an image | ||
| to produce clear images | ||
| complicated and time-consuming | ||
| dead specimens | ||
| pass readily through the lightly stained parts | ||
| to be impregnated with sth. |
VII. Find synonyms among the pool of words:
| Pool of words | Synonyms |
| 1)1. multiply /2. sample /3.increase /4. specimen | |
| 2)1. image / 2. visible / 3. apparent / 4. picture | |
| 3)1.beam/2.refract/3.examine/4.bend/5.explore/6. ray | |
| 4)1.organism/2.compound/3.living thing/4. complex |
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: