Unit 1 AS biology- microscopes

light & electron

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Basic Info

RESOLUTION: the capability of distinguishing between 2 seperate points (when you can no longer can see both points this means it is out of focus). The limit of resoultion is about 1/2 the wavelength of the radiation used to view the specimen.

Magnification= Size of image
                     Actual size of specimen

Measurements in cell biology:
1cm= 10 mm
1mm= 1000ym (micron)
1 ym = 1000 nm (nanometre)




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Light Microscopes

  • Poor resolution due to wavelenght of light (200 nanometres)
  • Image = true colours (using light)
  • Specimens- can be living or dead
  • Use biological stains - iodine, methyline blue
  • Attach to certain groups- protein, fats etc..
  • 1930's microscopes changed to electrons instead of light.
  • Focusing- lenses
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TEM= Transmission electron microscope.
1.consists of an electron gun that produces a beam of electrons that is focused on the specimen by a condenser electromagnet.
2. Parts of the specimen absorb electrons & therefore appear dark. Others let it through therefore appear light.
3.An image is produced on the screen & can be photographed to give a PHOTOMICROGRAPH.
4. Resolving power of a TEM is 0.1 nm
5. Problems with preparing specimen mean it can't always be achieved.

  • Image monocolour (light & dark) can add false colour
  • image may contain artefacts due to preperation
  • TEM can use heavy salts to stain & specimens must be really thin.
  • Complex staining
  • More expensive
  • specimens- dead becaused it's in a vacum
  • Resolution (better than light) 1-2 nanometres
  • Focusing- electromagnet
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Scanning electron Microscope produces a 3D image.
Scattering electrons & patterns determines on surface etc...
Same limitations as TEM but the samples don't have to be as thin, also it produces a 3D image.

SEM lower resolving power 20 nm but still 10 X better than Light!

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Which exam board is this for,could you please tag it to that exam board,thanks.

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