3.2- The electron microscope

Question 1

Why do electron microscopes have high resolving power?

Answer 1

Electron beam has a short wavelength

Question 1

What are two advantages of electron microscopes?

Answer 1

1. High resolving power
2. Electrons are negatively charged and so can be focused using electromagnets

Question 2

How does a transmission electron microscope (TEM) work?

Answer 2

Electron gun produces a beam of electrons that is focused onto the specimen by a condenser electromagnet.

Question 2

Why do areas of the specimen appear dark?

Answer 2

They have absorbed electrons

Question 2

Why do areas of the specimen appear bright?

Answer 2

Electrons have passed through it

Question 3

What is the resolving power of a TEM?

Answer 3

0.1nm

Question 4

Why is the resolving power not always achieved in practice?

Answer 4

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

Question 5

What are the limitations of the TEM?

Answer 5

1. The whole system must be in a vacuums so living specimens cannot be observed.
2. The specimen must be very thin
3. Complex staining process
4. Image is in black and white
5. The image may contain artefacts

Question 5

Why do specimens have to be kept in a near vacuum to be viewed effectively?

Answer 5

Because electrons would be absorbed or deflected by the air if not kept in a vacuum.

Question 6

How does a scanning electron microscope (SEM) WORK?

Answer 6

A beam of electrons is directed onto the surface of the specimen from above.
The beams then passed back and forth across a portion of the specimen in a regular pattern

Question 7

What is the resolving power of an SEM?

Answer 7

Around 20nm

Question 8

What are the limitations of the SEM?

Answer 8

Same as TEM but the specimen doesn’t have to be very thin because electrons do not penetrate