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Principle of Light Microscopy Light microscope is also called an optical microscope. It is an instrument which uses the visible light to enlarge the tiny specimens through microscopic lens. There are two types of light microscope, simple microscope has one lens with low magnification and the compound microscope has two lenses (objective and eye piece) with higher magnification. Parts of light microscope 1. Ocular/Eyepiece is present towards the viewer. It converts the real image of object into virtual image. It may be a combination of more than two lenses. 2. Lens tube is of about 160 mm length and holds the eyepiece. 3. Objective lens is present towards the object. It assembles the light reflected from the object at a place and produces a real image of object. 4. Objective revolver consists of objective lenses of different magnifying power. 5. Stand is attached with all components of microscope and it keeps them together. 6. Clip holds the object in place. 7. Stage is the place where object is kept for observing. 8. Condenser focuses the rays of sunlight on the object. 9. Fine and coarse adjustments control the distance between object and objective. They are used to produce sharp image of object. 10. Luminous-field diaphragm controls the diameter of light coming from light source. 11. Light source is the LED bulb used to illuminate the object. The older microscopes used concave mirror to focus the sunlight on the object to make it visible. 12. Base supports the microscope. Working principle- The light source illuminates the object. The first image of the object is formed by objective lens which is a real, inverted and magnified image. The image formed by the objective functions as the object for eyepiece, which produces the final, virtual and magnified image. In this way, the final image produced is inverted with respect to the object. Light microscope (Image Source- Microscopeworld) Principle of Electron Microscopy The first electron microscope was made by a German engineer, Ernst Ruska in 1931. It uses the beam of accelerated electrons to visualize the specimens and has a high resolution. Parts of Electron Microscope 1. Electron gun is heated tungsten filament which produces the beam of electrons. 2. Electromagnetic lenses- Condenser lens directs the electron beam towards the specimen. The electron beam coming out from specimen goes through objective lens. The projector or ocular lens produces the final magnified image. 3. The specimen holder is made up of thin layer of carbon held by metal grid. 4. The final image is obtained on fluorescent screen, below which a camera is present to capture the image. The working principle of electron microscope is discussed as follows: The beam of electrons are produced from the electron gun is focused on the specimen through two sets of condenser lens. An accelerating potential is applied between filament and anode to move the electrons downwards. The specimen to be observed is kept as thin section of 20-100 nm on specimen holder. The beam of electrons goes through specimen; the denser regions scatter more electrons on the screen than the lighter regions. The beam of electrons scattering from specimen passes through objective lens to create magnified image which is further magnified by ocular lens. Electron Microscope (Image Source- Microbenotes)
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