The electron microscopes

Question 1

What are the 2 advantages of electron microscopes?

Answer 1

• The electron beam has a very short wavelength so microscopes have a higher resolution.
• Electrons are negatively charged so the beam can be focused on using electromagnets.

Question 2

What are the 2 types of electron microscopes?

Answer 2

• TEM - Transmission electron microscope.

- SEM - Scanning electron microscope.

Question 3

What is a TEM?

Answer 3

TEM consists of an electron gun which produces a beam of electrons that is focused on the specimen by a condenser electromagnet.

Question 4

How does a TEM produce an image (photomicrograph?)

Answer 4

• Parts of the specimen absorbs the electrons so it appears darker and parts let the electrons to pass through so it appears to be lighter, therefore, an image can be produced.

Question 5

The resolving power of a TEM is 0.1nm but it cannot always be achieved why?

Answer 5

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

Question 6

What are the limitations of TEM?

Answer 6

• The whole system must be in a vacuum so living organisms cannot be observed.
• A complexed staining stage is required and the image produced isn’t even coloured.
• The specimen must be extremely thin.
• The final image might contain artefact (things that are resulted from the way when the specimen were prepared), the artefact might not be part of the natural organelle but we can’t tell from the image if it actually exists or not.

Question 7

Why does TEM require a thin specimen?

Answer 7

• Allow electrons to penetrate so it results in a flat 2D image to be produced. This can overcome by looking at many photomicrographs to build a 3D image but it’s slow and complicated so SEM is produced to overcome this.

Question 8

What is the difference between TEM and SEM?

Answer 8

• All the limitations to TEM also apply to SEM but SEM does not require the specimen to be extremely thin as electrons do not need to penetrate.

Question 9

How does SEM work?

Answer 9

SEM directs a beam of electrons on to the surface of the specimen from above and the beam is passed back and forth across a portion of the specimen and through this, a 3D image can be built up.
SEM has a lower resolution than TEM, 20nm but it’s still x10 better than the light microscope.