History of the microscope
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  • 1. History of the Microscope Electron Microscope By Mary Bellis, About.com Guide Electron Microscope < Introduction: History of Early Light Microscopes The introduction of the electron microscope in the 1930's filled the bill. Co-invented by Germans, Max Knoll and Ernst Ruska in 1931, Ernst Ruska was awarded half of the Nobel Prize for Physics in 1986 for his invention. (The other half of the Nobel Prize was divided between Heinrich Rohrer and Gerd Binnig for the STM.) In this kind of microscope, electrons are speeded up in a vacuum until their wavelength is extremely short, only one hundred-thousandth that of white light. Beams of these fast-moving electrons are focused on a cell sample and are absorbed or scattered by the cell's parts so as to form an image on an electron-sensitive photographic plate. Power of the Electron Microscope If pushed to the limit, electron microscopes can make it possible to view objects as small as the diameter of an atom. Most electron microscopes used to study biological material can "see" down to about 10 angstroms--an incredible feat, for although this does not make atoms visible, it does allow researchers to distinguish individual molecules of biological importance. In effect, it can magnify objects up to 1 million times. Nevertheless, all electron microscopes suffer from a serious drawback. Since no living specimen can survive under their high vacuum, they cannot show the ever-changing movements that characterize a living cell. Light Microscope Vs Electron Microscope Using an instrument the size of his palm, Anton van Leeuwenhoek was able to study the movements of one-celled organisms. Modern descendants of van Leeuwenhoek's light microscope can be over 6 feet tall, but they continue to be indispensable to cell biologists
  • 2. because, unlike electron microscopes, light microscopes enable the user to see living cells in action. The primary challenge for light microscopists since van Leeuwenhoek's time has been to enhance the contrast between pale cells and their paler surroundings so that cell structures and movement can be seen more easily. To do this they have devised ingenious strategies involving video cameras, polarized light, digitizing computers, and other techniques that are yielding vast improvements in contrast, fueling a renaissance in light microscopy. As most people know, optical microscopes are built based on expanding the principle of the magnifying glass, which magnifies objects using light and a lens. These instruments make it possible to see microscopic worlds by magnifying their visualization several hundreds or thousands of times that of the original size. However, there is a limit to the level of fineness (resolution) that is visible using light. It is not possible to observe objects that are smaller than the wavelength of visible light. On the other hand, electron microscopes use "electron beams," which have wavelengths much shorter than that of light. These apparatuses emit an electron beam toward the object to be investigated, detect the electrons which pass through, are reflected from or emitted from the object, and create a picture. The brighter and finer the electron beam, the higher the level of observation of the object’s internal details including atomic arrangement. • Optical Microscope Resolution, or more precisely point resolution, refers to the shortest distance in which two points can be recognized as two points. The resolution limit of the optical microscope is determined by the wavelength of visible light; that is, light that can be seen by the human eye. The wavelength of visible light is 400-700 nanometers (nm).The resolution is calculated to be approximately 100
  • 3. nm. On the other hand, the wavelength of electrons is less than 1/100,000 of that of visible light, or 1 picometer (pm), which is 0.001 nm. Therefore, theoretically, the resolution of electron microscopes can be less than several picometers. However, the resolution obtainable for an electron microscope is restricted to approximately 100 pm by lens aberrations. 1. A convex lens converge the incident parallel light beams. Optical microscopes enlarge images utilizing this convex lens function. 2. Electron microscopes, on the other hand, use electrons to enlarge images, employing an electron lens. The electron lens uses a magnetic field generated by an electrical current in a coil to converge the electrons. 3. Now, how are the electrons being converged in the magnetic field generated by the coil? 4. To make the explanation easy to understand, let us expand the coil.
  • 4. 5. When electrical current is passed through the coil… 6. A magnetic field is generated. 7. Put electrons in the magnetic field; then they travel parallel to the optical axis. 8. These electrons are influenced by the magnetic field
  • 5. 9. In this figure, the direction of the magnetic field is downward. The electrons are moving toward the back; this means that the electrical current is towards the front. 10. Let B the magnetic field and I the electrical current, then electrons are subject to the force F according to the Fleming’s left-hand rule. 11. This means that electrons are circling around the optical axis. and 12. moving toward the back.
  • 6. 13. When the electrons rotate, they are now subjected to the magnetic field parallel to the optical axis. 14. Since the direction of this force is toward the optical axis…, 15. the electrons move along the optical axis, spiraling toward it. 16. As a result, the electron path inside the coil is bent towards the optical axis.
  • 7. 17. Since the magnetic field becomes weaker, when an electron gets closer tothe optical axis, the bending of the electron path becomes smaller .... 18. As a result of this process, all electrons converge at a single point. 19. This is the lens function of the magnetic field generated by the coil. 20. In this way, it becomes possible to enlarge images by electrons in place of light. This mechanism is called an electron lens.
  • 8. 21. Actually, electron lenses… 22. are comprised of doughnut-shaped coils in iron casings. 23. The electron microscope lens system is made up of several electron lenses. 24. In this way, electron microscopes can observe much maller objects that cannot be seen by optical microscopes.