The Electron Microscope

Question 1

what are the advantages of the electron microscope?

Answer 1

• the electron beam has a very short wavelength and the microscope can therefore resolve objects well so it has a high resolving power
• as electrons are negatively charged the bean can be focused using electromagnets

Question 2

What is a disadvantage of using TEM to produce 3D images?

Answer 2

-it is a slow and complicated process

Question 3

What do electron microscopes require to work?

Answer 3

• because electrons are absorbed or deflected by the molecules in the air
• a bear vacuum has to be created within the chamber of an electron microscope in order for it to work efficiently

Question 4

what are the types of electron microscopes?

Answer 4

• transmission electron microscope (TEM)

- scanning electron microscope (SEM)

Question 5

why cant the resolving power of the TEM (0.1nm) be achieved in practice?

Answer 5

• it’s difficult to prepare the specimen which limits the resolution that can be achieved
• a higher energy electron beam is required and this may destroy the specimen

Question 6

what are the limitations of the TEM microscope?

Answer 6

• 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 as a result of specimen preparation
• these artefacts may appear on the finished photomicrograph but are not part of the specimen and can confuse researchers

Question 7

what type of image do TEM microscopes produce?

Answer 7

• the specimen must be extremely thin to allow electrons to penetrate which results in a flat 2D image
• if you take a series of sections through the specimen you can build up a 3D image of the specimen by looking at the series of photomicrographs produced

Question 8

What are the limitations of the SEM microscope?

Answer 8

• 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 as a result of specimen preparation
• these artefacts may appear on the finished photomicrograph but are not part of the specimen and can confuse researchers

Question 9

What can the best electron microscopes do?

Answer 9

-resolve objects that are just 0.1nm apart which is 2000 times better than a light microscope

Question 10

What is the TEM microscope?

Answer 10

• the TEM 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 the specimen absorb electrons which appears dark other parts allow electrons to pass through so appear brighter
• the image is produced on a screen and this can be photographed to give a photomicrograph

Question 11

What is the SEM microscope?

Answer 11

• the SEM directs a beam of electrons on the surface of the specimen from above rather than penetrating it from below
• the beam is passed back and fourth across the portion of the specimen in a regular pattern
• the electrons are scattered by the specimen and the pattern of this scattering depends on the computer analysis of the pattern of scattered electrons and secondary electrons produced
• the basic SEM has a lower resolving power than a TEM around 20nm but is still ten times better than a light microscope