Microscopy

Have you ever wished you could see little bit better - perhaps to read what someone else is writing from a distance or to recognize who exactly it is in the crowd at a football match? How frustrating it can be when you can't quite make out the print of the newspaper of the person opposite you on the train. This is how early scientists probably felt when they strained their eyes to decipher the detailed structure of organisms.

The light microscope opened up a new world of structural detail for biologists, but the wavelength of light limits the light microscope to distinguishing objects which are 0.2 µm or further from each other. This ability to distinguish two objects close together rather than to see them as a single image is called the resolving power or resolution.

So in 1933 the electron microscope was developed. This instrument works on the same principles as the light microscope except that instead of light rays, with their wavelengths in the order of 500 nm, a beam of electrons of wavelength 0.005 nm is used. This means that the electron microscope not only magnifies objects more, it also has a resolving power many thousands of times better than a light microscope.

Whereas the light microscope uses glass lenses to focus the light rays, the electron beam of the electron microscope is focused by means of powerful electromagnets. The image produced by the electron microscope cannot be detected directly by the naked eye. Instead, the electron beam is directed on to a screen from which black and white photographs, called photoelectronmicrographs, can be taken.

Light microscope	Electron microscope
Advantages	Disadvantages
Cheap to purchase and operate	Expensive to purchase and operate
Small and portable – can be used almost anywhere	Very large and must be operated in special rooms
Unaffected by magnetic fields	Affected by magnetic fields
Preparation of material is relatively quick and simple, requiring only a little expertise	Preparation of material is lengthy and requires considerable expertise and sometimes complex equipment
Material rarely distorted by preparation	Preparation of material may distort it
Natural colour of the material can be observed	All images are in black and white
Disadvantages	Advantages
Magnifies objects up to 2000×	Magnifies objects over 500 000×
The depth of field is restricted	It is possible to investigate a greater depth of field

Unit 4. Colonial organization

Did you know?

Ninety-nine per cent of all the

animal species that have ever

lived are now extinct.

Essential vocabulary

chickenpox to associate analogous

conjugation to benefit autotrophic

consequence to cure contaminated

cluster to devise extinct

diffraction to embrace flexible

droplet to estimate homologous

drought to fit immune

duplication to incorporate malignant

fever to indicate slim

fraction to invade tiny

genus to lack

grade to manage

hierarchy to prevent

host to reflect

immunity to respond

measles to split

multiplication to spread

parasite to transmit

rod

serum

Аналитическое чтение.

Did you know?

At a conservative estimate

there are 13.6 million species

on our planet.