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COPY LO:
To understand how the electronmicroscope works.
Starter
Write down whatis meant byresolution
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BATS
Describe how electron microscopes work
Explain the differences between transmissionand scanning electron microscopes
Describe the limitations of the transmissionand the scanning electron microscopes
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Light microscopes have poor resolutionbecause of the relatively long wavelength
of light.
B y u s i n g m o r e l e n s e smicroscopes can magnify by alarger amount, but this doesn't
always mean that more detailcan be seen.
The amount of detail dependson the resolving power of amicroscope, which is the
smallest separation at whichtwo separate objects can bedistinguished (or resolved).
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The resolving power of amicroscope is ultimately
limited by thewavelength of light
(400-600nm for visiblelight).
To improve the resolvingpower a shorter
wavelength of light isneeded, and sometimesmicroscopes have bluefilters for this purpose
(because blue has theshortest wavelength of
visible light).
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The Electron Microscope
Developed in 1930s
The electron beam has a veryshort wavelength - thereforeit has a high resolving power
The electrons are
negatively charged so the
beam can be focussed usingelectromagnets
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Modern Electron Microscopes - canresolve ob ects that are ust 0.1 nm a art
2000 timesbetter than
light
microscope
Electrons are
absorbed by
molecules in the air so
a near vacuum has to
be created in the
chamber of the
electron microscope
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2 Main types of Electron Microscopes
1. The transmissionelectron microscope(TEM)
2. The scanning electronmicroscope (SEM)
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Transmission Electron Microscope
Using books writedown as many facts
as you can about
the electron
microscope.
What are the main
differences
between the two?
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Transmission Electron Microscope
Here the beam passesthrougha thin section
of the specimen
Part of the specimen
absorb electrons and
appear dark
An image is produced on ascreen which is photographed
to give a photomicrograph
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Resolving power of TEM = 0.1 nm
Limitations:
- The whole system must be in avacuum
- A complex staining processisrequired
- Specimen must be extremely thin
- Specimen may contain artefacts
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Limitations:
- The whole system must be in a
vacuum - no living specimens
- A complex staining process isrequired - even then only black andwhite
- Specimen must be extremely thin
- Specimen may contain artefacts -things that result from the way thespecimen is prepared. We cannot
always be sure that what we see on aphotomicrograph really exists in thatform
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How might we build
up a 3 D image?
Take a series of
sections through a
specimen
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Rat Liver Cells
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Scanning Electron Microscope
Same as forTransmission Electron
Microscope except
specimen does not have
to be so thin
Beam of Electrons are
aimed at the surface
of the specimen andthe specimen scatters
them
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A computer analyses it and builds upa 3D image
Basic SEM has lower
resolving power than
a TEM - about 20nm
(still 10 times betterthan a light
microscope)
Ragweed pollen
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Summary Questions -
Page 45
Exam Questions
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Homework
References
Glenn Toole, Susan Toole (2008).A2 Biology. Cheltenham: Nelson
You Tube clips to watch
1. Revise for Test next week
2.