section 1: cell structure - topic 4: how microscopes work

Question 1

what is magnification?

Answer 1

how much bigger the image is than the specimen.

Question 2

what is the equation for magnification?

Answer 2

magnification = image size / object size

Question 3

what is resolution?

Answer 3

resolution is how detailed the image is - the minimum distance between 2 objects where they can be differentiated as separate.

Question 4

what are the 4 types of microscopes?

Answer 4

light.
laser scanning confocal.
transmission electron.
scanning electron.

Question 5

what do light microscopes use?

Answer 5

light.

Question 6

what is the resolution of light microscopes?

Answer 6

they have a lower resolution of electron microscopes.
they have a maximum resolution of about 0.2 micrometers.

Question 7

what are light microscopes used to look at?

Answer 7

whole cells or tissues.

Question 8

what are laser scanning confocal microscopes?

Answer 8

they are a special type of light microscope that use laser beams to scan a specimen that’s usually tagged with fluorescent dyes.

Question 8

what is the maximum useful magnification of a light microscope?

Answer 8

about x1500.

Question 9

how do laser scanning confocal microscopes work?

Answer 9

a laser beam is focused through a lens which is aimed at a beam splitter.
this splits the beam and some of the light is directed to the specimen.
when the laser hits the dyes, it causes them to give off fluorescent light.
this light is then focused through a pinhole onto a detector.
the detector is hooked up to a computer, which generates an image.
the pinhole means that any out-of-focus light is blocked, so these microscopes produce a much clearer image than a normal light microscope.

Question 10

what are laser scanning confocal microscopes used to look at?

Answer 10

they can be used to look at objects at different depths in thick specimens.

Question 11

what kind of pictures do laser scanning confocal microscopes produce?

Answer 11

multiple images produced by the microscope can be combined by the computer to generate 3D images of a specimen.

Question 12

what do electron microscopes use?

Answer 12

electrons.

Question 13

what is the resolution of electron microscopes?

Answer 13

they have a higher resolution than light microscopes, so give more detailed images.

Question 14

what are the 2 types of electron microscopes?

Answer 14

TEM.
SEM.

Question 15

how do TEMs work?

Answer 15

they use electromagnets to focus a beam of electrons, which is then transmitted through the specimen to produce 2D images.
denser parts of the specimen absorb more electrons, which makes them look darker on the image you end up with.

Question 16

why are TEMs good?

Answer 16

because they provide high resolution images, so can be used to look at very small organelles i.e. ribosomes.
can also be used to look at the internal structures of organelles in detail.

Question 17

limitations of TEMs?

Answer 17

specimens viewed on TEMs need to be quite thinly sliced.
the angle at which specimens are cut can affect how they appear.

Question 18

how do SEMs work?

Answer 18

they scan a beam of electrons across the specimen.
this knocks off electrons from the specimen, which are gathered in a cathode ray tube to form an image.

Question 19

strengths of SEMs?

Answer 19

the images produced show the surface of the specimen & can be 3D.

Question 20

limitations of SEMs?

Answer 20

they give lower resolution images than TEMs.

Question 21

how do you prepare samples for electron microscopes?

Answer 21

they are treated with a solution of heavy metals (like lead).

the metal ions act to scatter the electrons that are fired at the sample and give contrast between different structures.

Question 22

what do images produced by electron microscopy look like?

Answer 22

they are always black & white, although colour can be added to images after they’ve been made to make them easier to interpret.

Question 23

summary of max. resolution & max. magnification.

Answer 23

light: 0.2 micrometres
x1500
TEM: 0.0002 micrometres
can be more than x1,000,000
SEM: 0.002 micrometres
usually less than x500,000