3.2- THE ELECTRON MICROSCOPE Flashcards
Why do light microscopes have poor resolution?
due to relatively long wavelength of light
What was developed in the 1930s?
microscope that used a beam of electrons instead of light
What was the microscope that used a beam of electrons instead of light called?
electron microscope
What two main advantages did the electron microscope have? (2)
electron beam has very short wavelength and microscope can therefore resolve objects well- has high resolving power
as electrons negatively charged beam can be focused using electromagnets
To what resolution can the best modern electron microscope able to resolve objects?
objects that are just 0.1nm apart
How many times better is the resolution of electron microscopes than light microscopes?
2000 times better than a light microscope
Why must a near-vacuum be created within the chamber of an electron microscope for it to work efficiently?
as electrons are absorbed or deflected by molecules in air
What are the two types of electron microscope?
transmission electron microscope (TEM)
scanning electron microscope (SEM)
What does the TEM consist of?
electron gun that produces a beam of electrons that’s focused onto specimen by condenser electromagnet
What happens in a TEM?
beam passes through a thin section of the specimen
What does parts of the specimen absorb?
electrons
What happens when parts of the specimen absorb electrons?
appear dark
What happens in other parts of the specimen?
they allow electrons to pass through and so appear bright
What is the image that’s produced on a screen and can be photographed called?
photomicrograph
What is the resolving power of the TEM?
0.1nm
Why cannot the resolving power of the TEM not always be achieved? (2)
difficulties in preparing specimen limit resolution that can be achieved
higher energy electron beam required and this may destroy specimen
What are the main limitations of the TEM? (3)
whole system must be in vacuum so living specimens cannot be observed
complex ‘staining’ process required + even then image not in colour
image may contain artefacts
What are artefacts?
things that result from way specimen prepared
Where may artefacts appear?
on finished photomicrograph but not apart of natural specimen
Why must the specimen be extremely thin in the TEM?
to allow electrons to penetrate
As the specimen in TEM must be extremely thin, what does this result in?
flat, 2D image
How can we partly get over the flat, 2D image produced by TEM?
taking a series of sections through a specimen
build up 3D image by looking at series of photomicrographs produced
However what is the process of building up a 3D image from the TEM described as?
slow and complicated
How can the problem of producing a 3D image be overcome?
through the development of the SEM
What limitatiosn does the SEM have?
same limitations to TEM, but specimens need not be extremely thin
Why does the specimen not have to be extremely thin in SEM?
electrons don’t penetrate
How does the SEM work?
directs beam of electrons on to surface of specimen from above, rather than penetrating it from below
What does the beam do in SEM?
beam passed back and forth across portion of specimen in regular pattern
How are the electrons scattrered in SEM?
by specimen
What does the scattering of electrons in SEM depend on?
contours of specimen surface
How can a 3D image from SEM be built?
by computer analysis of pattern of scattered electrons and secondary electrons produced
What is the resolving power of a basic SEM like in comparison the TEM?
lower resolving power, around 20nm
How many times better is the resolving power of the TEM than the light microscope?
ten times better
