microscopes

Question 1

define magnification

Answer 1

• how many times bigger the image is compared to the object

Question 2

define resolution

Answer 2

ability to distinguish between two points

Question 3

describe an optical (light) microscope

Answer 3

• used visible light to pass and bend through lens system to form an image
• illuminate specimens and produce magnified image
• specimen can be alive
• typically individual cells are transparent + components are visible if coloured with special stain (–> will kill the cells)
• can observe eukaryotic cells, nuclei, mitochondria and chloroplasts

max resolution = 0.2 micrometres
max mag = x1500
wavelength of visible light = 500-650

Question 4

what are the advantages of light microscopes

Answer 4

• accessible + affordable => commonly used in labs
• can observe living and transparent specimens
• doesn’t need complex prep

Question 5

what are the limitations of light microscopes?

Answer 5

• limited resolution bc of short wavelengths
• cannot observe structures smaller than wavelength of light + distinguish between objects closer than half this value
• smaller mag + res

Question 6

what is an electron microscope ?

Answer 6

• uses beams of electrons
• higher mag + res power -=> more detail seen
• max mag = x1500000
• max res = 0.2nm

Question 7

what is a transmission electron microscope?

Answer 7

• uses electromagnets to focus beams of electrons
• electrons penetrate cell –> details of cells internal structures
• denser parts of specimen absorbs more electrons –> appear darker on final image

Question 8

what are advantages of TEM?

Answer 8

• They give high-resolution images (more detail)
• This allows the internal structures within cells (or even within organelles) to be seen
• enable detailed examination of internal cellular structures and ultrafine details

Question 9

what are the limitations of TEM?

Answer 9

• expensive
• cannot be used to observe live specimens (as there is a vacuum inside a TEM, all the water must be removed from the specimen and so living cells cannot be observed
• only be used with very thin specimens or thin sections of the object
• lengthy complex treatment required to prepare specimens means that artefacts can be introduced
• do not produce a colour image
• require dedicated facility
• limited sample penetration due to electron beam absorption

Question 10

what are scanning electron microscopes?

Answer 10

• a beam of electrons across the specimen
• beam bounces off the surface of the specimen -> electrons are detected, forming an image
  = SEMs can produce three-dimensional images that show the surface of specimens
• resolving power = 0.3-10nm
• samples have to be coated in heavy gold ions

Question 11

what are the advantages of SEMs?

Answer 11

• high resolution images + excellent depth of field
• They can be used on thick or 3-D specimens
• They allow the external, 3-D structure of specimens to be observed
• wide magnification range

Question 12

what are the limitations of SEMs?

Answer 12

• expensive
• lower resolution images (less detail) than TEMs
• cannot be used to observe live specimens
• do not produce a colour image
• require dedicated facility
• time consuming - require special techniques
• samples must be conductive + coated with conductive metal = limits certain applications