3.2 The electron microscope
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What are the two main advantages of the electron microscope over the light microscope
. The electron beam has a short wavelength and the microscope can therefore resolve objects well, so it has a high resolving power . All electrons are negatively charged so the beam can be focussed using electromagnets
What are the two types of electron microscope
. The transmission electron microscope TEM . Tne scanning electron microscope
Describe the transmission electron microscope
. The TEM consists of an electron gun that produces a beam of electrons that is focused onto the specimen by a condenser electromagnet. . In a TEM the beam passes through a thin section of the specimen . Parts of this specimen absorb electrons so appear dark and other parts allow electrons to pass through so appear bright. . An image is produced on a screen and this can be photographed to give a photomicrograph.
The resolving power of the TEM is 0.1nm although this cannot always be achieved in practice, why?
. Difficulties preparing the specimen limit the resolution that can be achieved . A higher energy electron beam is required and this may destroy the specimen
What are the main limitations of TEM (transmission electron microscope)
. The whole system must be in a vacuum and therefore living specimens cant be observed . A complex staining process is required and even then the image isn’t in colour . The specimen must be extremely thin . The image may contain artefacts (things that result from the way the specimen is prepared) Artefacts may appear on the finished photomicrograph but are not part of the natural specimen so its not always easy to be sure that what we see on a photomicrograph really exists in that form
What type of image is a result of TEM
. A 2D flat image however we can get over this and build up a 3D image from taking series of sections through the specimen and looking at the photomicrographs produced
What is the SEM (scanning electron microscope)
The SEM directs a beam of electrons onto the surface of a specimen from above rather than penetrating it from below The beam is passed back and forth across a portion of the specimen in a regular pattern The electrons are scattered by the specimen and the pattern of this scattering depends on the contours of the specimen surface We can build up a 3D image by computer analysis of the pattern of scattered electrons and secondary electrons produced.
What is the difference in resolving power between the SEM and TEM
The basic SEM has a lower resolving power than TEM, around 20nm but is still 10 times better than a light microscope
Why do electron microscopes have to be kept in a vacuum
The electron bean that is produced will be absorbed by air As a result only non living specimens can be examined