Microscopes

For OCR AS Level Biology - Unit F211 - microscopes

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  • Created by: Rean
  • Created on: 03-10-09 19:24

Magnification and Resolution

Magnification - Number of times greater an image is than the actual object.

Magnification= Image size vvghorggggg The length of the scale bar

gggghghghhghActual object size gghhghfThe length the scale bar represent

Actual object size = Image size

jjjgjgjgjhghjgjgjgjMagnification

Resolution - Ability to distinguish between two separate points; the degree of detail that can be seen in an image.

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Units of measurement

Micrometres and nanometres

  • 1µm (micrometre) = 1/1000mm or 1 x 10−3 mm or 1 x 10−6 m

To get from mm to µm multiply by 1000

  • 1nm (nanometre) = 1/1000µm or 1 x 10−6 mm or 1 x 10−9 m

To get from µm to nm multiply by 1000

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Light Microscopes

Light Microscopes - Use glass lenses to refract light rays and produce a magnified image of an object

Eyepiece lens magnifies and focuses the image from the object onto the eye.

Objective lens collects light passing through the specimen and produces a magnifies image.

Condenser lens focuses the light onto the specimen held between the cover slip and slide.

Condenser Iris diaphragm is closed slightly to produce a narrow beam of light.

  • Specimens need to be thin and stained to show up clearly.
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TEM

Transmission Electron Microsopes (TEM) - pass beams of electrons through a specimen to produce images of an object.

Electron gun and anode - produce a beam of electrons

Condenser Electromagnetic lens - directs the electron beam onto the specimen

Grid - specimen is placed on it

Objective electromagnetic lens - produces an image

projector electromagnetic lenses - focus the magnified image onto the screen

Screen or photographic plate - shows the image of the specimen

  • Electrons have shorter wavelengths than light rays so give a much higher resolution.
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SEM

Scaning Elctron Microscopes (SEM) - Bounces electron beams off of the surface of an object giving a 3D image. Allows organelles to be seen.

  • The original images produced by an electron microscope are in black, white and grey only but false colours are often added using a computer to help identify diffent structures.
  • Specimen needs to be thin and stained.
  • In electron microscopy stains are heavy metals e.g lead or osmium.
  • The ions are large and positively charged. the negatively charged electrons do not pass through cells and therefore do not show up on the screen.
  • Ions of the metals are taken up by some parts of the cell more than others. Structures which have taken up the stain appear darker than other areas
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Comparisons

Type vhuygjhHighest Magnification ggjjjjgjkgBest Resolution

Light hghjghgjggx 2000 ghghgfhgfhfghfhgfhgghf300nm

TEM hhhhhghhx 500,000 + (2D) hghghghhhhh0.1nm

SEM hgjhhjhghgx 25,000 (3D) jjhjhjhjfgguggjhgj10nm

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Calibrating a Microscope

Calibrating a microscope

Why? To measure the size of a structure under the field of view with a specific magnification

How? using an eyepiece graticule (a scale of unknow lengthinserted into an eyepiece) and using a stage micrometre (a scale of known length 10mm) placed onto the stage and superimposed on top of the eyepiece graticule.

  • To measure cells the graticule must first be calibrated using a micrometer
  • A stage micrometer is a microscope slide with a scale very accuractely etched into 100µm divisions.
  • The graticule has 100 "dinks" - find how many dinks on the micrometre fit into 100 on the gratucule. Then add µm onto the number as the unit of measurement
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