2. Microscopes Flashcards
define magnification
the ratio of an objects size to its real size
define resolution
the measure of the minimum distance of two distinguishable points
define contrast
visible differences in brightness or colour between parts of the sample
what is the most common light microscope and how does it work
bright field microscope, passes light through slide stained with a dye
what did Robert Hooke achieve
first to visualise a micro-organism under a light microscope
what did Anton Van Leeuwenhoek achieve
used single-lens microscopes, which he made, to make the first observations of bacteria and protozoa
list pros and cons of the light microscope
pros: ability to image living cells
cons: limited resolution
what is the resolution of a light microscope
0.2 microns
name 4 methods if sample preparation for light microscope
- whole mounts
- tissue section
- fixation
- staining
what does fixation of a slide achieve
uses chemical fixatives to prevent cell autolysis and preserve the structure of tissue
give an example of a stain
eosin
name 4 different types of light microscope
- bright field
- fluorescent
- phase
- confocal
how does a fluorescent microscope work
fluorescent substances absorb short-wavelength UV- this shows the locations of specific molecules
how does a phase microscope work
enhances the contrast in cells
how does a confocal microscope work, what is a limitation
uses lasers and special optics for ‘optical sectioning’
this generates 3D images of living cells
❌ very expensive
who developed the electron microscope in the 1930s
Ruska & Knoll
what type of light wavelength does the electron microscope use
short wavelength- generates a high resolution (0.08nm)
what is the resolution of an electron microscope
0.08 nm
why must an electron microscope be kept under vacuum
because electrons have poor penetrating power
describe how a transmission electron microscope works
electron gun produces electrons by thermionic emission
the beam passes through the specimen
the image is focused and magnified by lenses
a fluorescent screen converts the image so its visible
what is cryo-electron microscopy
where you rapidly freeze samples and put them under the microscope
describe how a scanning electron microscope works
the electron beam is scanned across the specimen
electrons are reflected from the surface of the specimen and are collected on a screen
gives a 3D image
what must be done with biological samples before being examined under SEM
must be fixed and dried
how are electron microscope slides prepared, explain
- fixation- in glutaraldehyde and then osmium tetroxide
2. dehydrated- water is replaced with ethanol
