Microscopes

• Resolution


  The microscopes ability to distinguish details of a sample or specimen
• In 1609, an italian scientist called Galileo Galilei developed his first compound microscope
• In the mid 17th century, Robert Hooke, the English father of microscopy made improvements to Galileo's design. He was the first person to observe and describe a cell
• In the mid 19th century, Charles A Spencer produced microscopes that could magnify objects up to 5000 times their normal size with blue light
• Light microscopes cannot be used to view objects that are smaller than half the wavelength of light. This is because the objects are not large enough to interfere with the passage of the light waves
• Light wavelength varies between 400nm (nanometers) for violet light and 700nm for red light
• The resolving power of a microscope refers to its ability to differentiate between closely spaced objects
• Transmission Electron Microscope

  • TEM allows electrons to be transmitted through the specimen before viewing
  • Sections that are more dense and absorb more electrons appear blacker in the image
  • Dense areas appear more black or dark on an electron graph
• Scanning Electron Microscope

  • IN SEM, specimens are frozen, fractured and coated in gold atoms
  • They are used to form a TV image on a cathode ray tube
  • A beam of electrons is scanned to and fro across the specimen. The electrons that are reflected off the surface area collected
• Advantages of TEM over SEM

  • Transmission electron microscope has a greater resolving power
  • TEM has a resolution of 0.5nm, whereas SEM has a resolution between 5nm and 20nm
• Advantages of SEM over TEM

  • Surface structures of specimens can be seen easily
  • Structures can be seen in 3D
  • Scanning electron microscopes can view much larger samples than the TEM can
• Limitations of electron microscopy

  • Expensive to buy and run
  • Preparations of material is time consuming, expensive and technical
  • Preservation and staining can damage tissue structure
• Magnification = Size of image

  Size of real object