3.2 - Electron microscope

• Why do electron microscopes have a higher resolution that optical microscopes ?


  The electron beam has a very short wavelength whereas the light has a longer wavelength meaning the electron microscope can resolve objects better
• What’s an advantage of the electron beam being negatively charged ?


  The beam can be focused using electromagnets
• Why do electron microscopes require a vacuum to work ?


  Electrons are absorbed or deflected by air molecules which could create distorted images
• 4 differences between the structure of an electron microscope and an electron microscope ?

  Instead of a light source it has an electron source, has a magnetic condenser instead of a condenser lens, has a magnetic objective instead of an objective lens, has a magnetic projector instead of the eyepiece lenses (projector)
• What are the 2 types of electron microscope ?

  Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM)
• How is the beam in a TEM produced ?


  An electron gun produces the beam and it’s then focused onto the specimen by a condenser electromagnet
• How does a TEM reveal the internal structures of a specimen ?


  Parts of the specimen absorb electrons and appear dark whereas other parts allow them through so appear bright
• 2 reasons why the max resolution of a TEM may not be reached ?


  Errors preparing the specimen, a high energy beam is required which can destroy the specimen
• What’s the image produced by a TEM called ?

  A photomicrograph
• 4 limitations of a TEM ?


  In a vacuum so living specimens can’t be observed, complex staining process is required and even then the image isn’t in colour, specimens must be very thin, image may contain artefacts which make is difficult to determine what actually exists in the specimen
• How does the SEM work ?


  Electrons pass back and forth across the specimen, these are scattered creating secondary electrons which are detected and a computer builds a 3D image of the specimen, the pattern of scattering depends upon the contours of the specimen surface
• 2 disadvantages of a SEM ?


  Lower resolution, in a vacuum so living specimens can’t be viewed
• An advantage of using a SEM over a TEM ?


  A thin sample isn‘t needed
• 2 advantages of using a TEM instead of a SEM ?


  Higher resolution, can view internal structures of a specimen