3.2 The electron microscope

Question 1

What are the benefits of using an electron microscope rather than a light microscope?

Answer 1

• the electron beam has a very short wavelength meaning it has a high resolving power
• since electrons are negatively charged, the beam can be focused using electromagnets

Question 2

How does a transmission electron microscope work?

Answer 2

A beam of electrons is produced and focused on a thin specimen by a condenser electromagnet. Parts of the specimen absorb the electrons and appear dark, other parts allow the electrons to pass through and appear bright. the result is a flat, 2D image.

Question 3

What are the main limitations of the TEM?

Answer 3

• specimen must be in a vacuum so live specimens cannot be observed
• complex staining process is required and image is still in black and white
• specimen has to be very thin
• image may include artifacts due to the way it is prepared.

Question 4

How does a scanning electron microscope work?

Answer 4

The SEM directs a beam of electrons onto the surface of the specimen from above, which is then passed back and forth across it in a regular pattern.
the specimen scatters the electrons in a pattern which reflects the contours of the object.
This produces a 3D image built up on a computor.

Question 5

what are the limitations of a SEM?

Answer 5

SEMs have all the same limitations as a TEM but the specimen doesn’t have to be as thin.
however, the resolving power is lower.