Electron Microscopes

Question 1

Explain what the TEM (Transmission Electron Microscope) is

Answer 1

• It works much like a light microscope, transmitting a beam of electrons through a thin specimen and then focusing the electrons to form an image on a screen or film
• Electrons pass through and around the specimen, giving some contrast - the image is 2d with a magnification of x500,000
• this is the most common form of an electron microscope and has the best resolution

Question 2

More details on the TEM (transmission electron microscope)

Answer 2

• Passes a beam through the specimen. The electrons that pass through the specimen are detected on a fluorescent screen on which the image is displayed
• thin sections of specimens are needed for transmission electron microscopy as the electrons have to pass through the specimen for the image to be produced
• this is the most common form of electron microscope and has the best resolution
• provides for detailed study of internal ultrastructure of cells

Question 3

Explain what the SEM (scanning electron microscope) is

Answer 3

• The SEM (scanning electron microscope) scans a fine beam of electron onto a specimen and collects the electrons scattered by the surface . This has poorer resolution but gives excellent 3-dimensional images of surfaces
• electrons do not go through or pass around the sample but reflect off - the image is 3d with a magnification of x100,000

Question 4

more details about SEM (scanning electron microscope)

Answer 4

• passes a beam of electrons over the surface of the specimen in the form of a scanning beam
• elects are reflected off the surface of the specimen as it has been previously coated in heavy metals (heavy metals are used to stain samples)
• it is these reflected electron beams that are focused of the fluorescent screen in order to make up the image
• larger, thicker structures can thus be seen under the SEM as the electrons do not have to pass through the sample in order to form the image. This gives excellent 3d images of surfaces
• however, the resolution of the SEM is lower than the TEM

Question 5

Why can’t electron microscopes see in colour and what can be done about it?

Answer 5

All electron microscopes cannot see in colour because there is only one wavelength transmitting (electrons)

Colour has to be educatedly guessed at & added

Question 6

You can magnify an image to make it larger but what doesn’t this always mean?

Answer 6

But it doesn’t always make it clearer

Question 7

When does the image go blurred when you continue to magnify?

Answer 7

When the microscope has reached a limit to its resolution (every microscope has this)

Question 8

How does an electron microscope have a much higher resolution than a light microscope?

Answer 8

Because electrons have a much shorter wavelength than light is an electron microscope has a much higher resolution

(This is because the shorter the wavelength of light, the higher the resolution)

Question 9

When does diffraction affect resolution?

Answer 9

As the light waves spread out or overlap

Question 10

Give the light microscope pros

Answer 10

• inexpensive to buy & operate
• small & portable
• simple sample preparation
• vacuum not required
• natural colour of sample is seen
• specimens alive or dead (can only be dead for electron)

Question 11

Give the negatives of an electron microscope?

Answer 11

• very large & must be operated in special rooms
• affected by magnetic and particles in the air
• preparation of material is lengthy, considerable expertise, and sometimes requires complex equipment
• preparation of material may distort it
• all images are in black or white
• expensive to purchase & operate