3.2 The electron microscope Flashcards
Why do light microscopes have poor resolution
Because of the relatively long wavelength of light
What are the two main advantages of electron micrscopes
- The electron beam has a very short wavelength—>higher resolution
- As electrons are negatively charged the beam can be focused using electromagnets
What are the two tpes of electron microscopes
Scanning and Tranmission electron micrscope
How does a Tranmission electron micrscope work
An electron gun produces a beam of electrons that is focused on to a specimen by an electromagnet
The beam passes through a thin section of the specimen
An image is produced onto a screen
Why are some areas darker on the specimen in the Transmission Electron Micrscope (TEM)
Some parts absorb more electrons therefore appear darker
Why are some areas brighter in the TEM
Some parts of the specimen allow the electrons to pass through
Although the resolving power of the TEM is 0.1nm why can this not always be achieveid
There could be diffuclties preparing the speicmen
A higher energy electron beam is required and this may destroy the specimen
What are the main limitations of the TEM(4)
- Must be in a vacuum therfore living specimens cannot be observed
- A complex staining process is required and it is not in colour
- Specimen must be extremely thin
- Image may contain artefacts
- The image will be flat and in 2D
What is one thing the Scanning electron micrscope can do that the TEM cannot do
Take 3D images
SEM
Has similar limiations to the TEM apart from the flat and 2D and the specimen does not need to be extremly as electrons do not penetrate
How does the SEM work
SEM directs a beam of electrons onto the surface of the specimen from above
This beam is then passed back and forth across a portion of the specimen in a regular pattern
The electrons are scattered by the specimen surface
