The electron microscope Flashcards
How are the electrons focused?
The electrons are a negatively charged beam that can be focused using electromagnets.
How much better are electron microscopes than light microscope?
The best electron microscopes have can resolve objects just 0.1nm apart, this is 2000 times better than light microscopes.
How do we get the electron microscope to work effectively?
Because electrons are absorbed or deflected by molecules in air, a near-vacuum has to be created within the chamber of the microscope.
What are the two types of electron microscope?
The transmission electron microscope and the scanning electron microscope.
What does a TEM consist of?
It consists of an electron gun that produces a beam of electrons that is focused onto the specimen by a condenser electromagnet. The beam passes through a thin section of the specimen, parts of this specimen allow electrons to pass through so appear bright.
Why can the resolving power of a TEM not always be achieved?
- 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 the TEM?
- The whole system must be in a vacuum and therefore living specimens cannot be observed
- A complex ‘staining’ process is required and even then the image is not in colour
- The specimen must be extremely thin
- The image may contain artefacts.
What is an artefact?
Things that result from the way the specimen is prepared. They may appear on the finished photomicrograph but are not part of the natural specimen.
Why must TEM specimens be extremely thin?
To allow electrons to penetrate.
What sort of image does a TEM produce?
2D image, we can look at photos in series to try picture a 3D model although this is slow and complicated.
Limitations of SEM?
- The whole system must be in a vacuum and therefore living specimens cannot be observed
- A complex ‘staining’ process is required and even then the image is not in colour
- The image may contain artefacts.
How does an SEM work?
It directs a beam of electrons on to the surface of the 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 because of the different contours on the surface.
Which has the higher resolving power, TEM or SEM?
TEM.
What sort of image does SEM produce?
A 3D image by computer analysis of the pattern of scattered electrons and secondary electrons produced.
