Microscopes

• Magnification


  How many times bigger the image produced by the microscope is than the real-life object you are viewing
• Resolution


  The ability to distinguish between objects that are close together (i.e. the ability to see two structures that are very close together as two separate structures)
• Types of microscopes


  • Optical microscopes (sometimes known as light microscopes)
  • Electron microscopes
• Optical (light) microscopes


  • Use light to form an image
  • Limited resolution due to the wavelength of light
• Optical microscopes cannot resolve (distinguish between) objects closer than half the wavelength of visible light (500-650 nanometres)
• Optical microscopes have a maximum resolution of around 0.2 micrometres (μm) or 200 nm
• Optical microscopes


  • Can be used to observe eukaryotic cells, their nuclei and possibly mitochondria and chloroplasts
  • Cannot be used to observe smaller organelles such as ribosomes, the endoplasmic reticulum or lysosomes
• The maximum useful magnification of optical microscopes is about x1500
• Electron microscopes


  • Use electrons to form an image
  • Much higher resolution than optical microscopes
• Electron microscopes have a maximum resolution of around 0.0002 μm or 0.2 nm (ie. around 1000 times greater than that of optical microscopes)
• Electron microscopes can be used to observe small organelles such as ribosomes, the endoplasmic reticulum or lysosomes
• The maximum useful magnification of electron microscopes is about *1,500,000
• Types of electron microscopes

  • Transmission electron microscopes (TEMS)
  • Scanning electron microscopes (SEMs)
• Transmission electron microscopes (TEMS)


  • Use electromagnets to focus a beam of electrons
  • Beam of electrons is transmitted through the specimen
  • Denser parts of the specimen absorb more electrons, appearing darker on the final image
• Advantages of TEMS


  • They give high-resolution images (more detail)
  • This allows the internal structures within cells (or even within organelles) to be seen
• Disadvantages of TEMS


  • They can only be used with very thin specimens or thin sections of the object being observed
  • They cannot be used to observe live specimens
  • The lengthy treatment required to prepare specimens means that artefacts can be introduced
  • They do not produce a colour image
• Scanning electron microscopes (SEMS)

  • Scan a beam of electrons across the specimen
  • The beam bounces off the surface of the specimen and the electrons are detected, forming an image
  • Can produce three-dimensional images that show the surface of specimens
• Advantages of SEMS


  • They can be used on thick or 3-D specimens
  • They allow the external, 3-D structure of specimens to be observed
• Disadvantages of SEMS


  • They give lower resolution images (less detail) than TEMS
  • They cannot be used to observe live specimens
  • They do not produce a colour image