3.2 The Electron Microscope

Question 1

Why do light microscopes have poor resolution

Answer 1

Because of their relatively long wavelength of light

Question 2

What are the two main advantages of electron microscopes

Answer 2

1. The electron beam has a very short wavelength and the microscope can therefore resolve things well. It has HIGH RESOLVING POWEr
2. As electrons are negatively charged, magnets can be used to direct the electron beam

Question 3

What are the two types of electron microscopes

Answer 3

1. Transmission electron microscope

2. Scanning electron microscope

Question 4

How do Transmission electron microscopes (TEMs) work

Answer 4

The TEM consists of an electron gun that produces a beam of electrons that is focussed onto the specimen by a condenser electromagnet.

In a TEM, the electron beam passes through a thin section of the specimen. Parts of the specimen absorb electrons and therefore appear dark, other parts dont and appear bright

A image is produced on a screen and this can be photographed to give a photomicrograph

Question 5

Why can the resolving power of a TEM not be reached

Answer 5

1. Difficulties preparing the specimen limit the resolution that can be achieved
2. A higher energy electron beam is required which may destroy the specimen

Question 6

What are the main limitations of TEM

Answer 6

1. The whole system must be in a vacuum, therefore living specimens cannot be observed
2. A complex ‘staining’ process is required and even then the image isnt in colour
3. The specimen must be extremely thin
4. The imagine may contain artefacts.
5. Must be thin so produces a flat, 2-D image. Can be made 3D by looking at a series

Question 7

How does a SEM work

Answer 7

Similar to a TEM, the SEM directs a beam of electrons on to the surface of the specimen from above, rather than penetrating from below.

The beam is then passed back and forth across a portion of the specimen in a regular pattern. The electrons are scattered by the specimen and the pattern of this scattering depends on the contours of the specimen surface

We can build up a 3-D image via computer analysis

Question 8

What are the limitations of a SEM

Answer 8

All the same as a TEM

HOWEVER

• SEM produces a 3D image without the slow process a TEM requires
• The object doesnt have to be thin