3.2- THE ELECTRON MICROSCOPE

Question 1

Why do light microscopes have poor resolution?

Answer 1

due to relatively long wavelength of light

Question 2

What was developed in the 1930s?

Answer 2

microscope that used a beam of electrons instead of light

Question 3

What was the microscope that used a beam of electrons instead of light called?

Answer 3

electron microscope

Question 4

What two main advantages did the electron microscope have? (2)

Answer 4

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

Question 5

To what resolution can the best modern electron microscope able to resolve objects?

Answer 5

objects that are just 0.1nm apart

Question 6

How many times better is the resolution of electron microscopes than light microscopes?

Answer 6

2000 times better than a light microscope

Question 7

Why must a near-vacuum be created within the chamber of an electron microscope for it to work efficiently?

Answer 7

as electrons are absorbed or deflected by molecules in air

Question 8

What are the two types of electron microscope?

Answer 8

transmission electron microscope (TEM)

scanning electron microscope (SEM)

Question 9

What does the TEM consist of?

Answer 9

electron gun that produces a beam of electrons that’s focused onto specimen by condenser electromagnet

Question 10

What happens in a TEM?

Answer 10

beam passes through a thin section of the specimen

Question 11

What does parts of the specimen absorb?

Answer 11

electrons

Question 12

What happens when parts of the specimen absorb electrons?

Answer 12

appear dark

Question 13

What happens in other parts of the specimen?

Answer 13

they allow electrons to pass through and so appear bright

Question 14

What is the image that’s produced on a screen and can be photographed called?

Answer 14

photomicrograph

Question 15

What is the resolving power of the TEM?

Answer 15

0.1nm

Question 16

Why cannot the resolving power of the TEM not always be achieved? (2)

Answer 16

difficulties in preparing specimen limit resolution that can be achieved

higher energy electron beam required and this may destroy specimen

Question 17

What are the main limitations of the TEM? (3)

Answer 17

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

Question 18

What are artefacts?

Answer 18

things that result from way specimen prepared

Question 19

Where may artefacts appear?

Answer 19

on finished photomicrograph but not apart of natural specimen

Question 20

Why must the specimen be extremely thin in the TEM?

Answer 20

to allow electrons to penetrate

Question 21

As the specimen in TEM must be extremely thin, what does this result in?

Answer 21

flat, 2D image

Question 22

How can we partly get over the flat, 2D image produced by TEM?

Answer 22

taking a series of sections through a specimen

build up 3D image by looking at series of photomicrographs produced

Question 23

However what is the process of building up a 3D image from the TEM described as?

Answer 23

slow and complicated

Question 24

How can the problem of producing a 3D image be overcome?

Answer 24

through the development of the SEM

Question 25

What limitatiosn does the SEM have?

Answer 25

same limitations to TEM, but specimens need not be extremely thin

Question 26

Why does the specimen not have to be extremely thin in SEM?

Answer 26

electrons don’t penetrate

Question 27

How does the SEM work?

Answer 27

directs beam of electrons on to surface of specimen from above, rather than penetrating it from below

Question 28

What does the beam do in SEM?

Answer 28

beam passed back and forth across portion of specimen in regular pattern

Question 29

How are the electrons scattrered in SEM?

Answer 29

by specimen

Question 30

What does the scattering of electrons in SEM depend on?

Answer 30

contours of specimen surface

Question 31

How can a 3D image from SEM be built?

Answer 31

by computer analysis of pattern of scattered electrons and secondary electrons produced

Question 32

What is the resolving power of a basic SEM like in comparison the TEM?

Answer 32

lower resolving power, around 20nm

Question 33

How many times better is the resolving power of the TEM than the light microscope?

Answer 33

ten times better