Chapter 2.1 Microscopes

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  • Chapter 2.1 Microscopes
    • Magnification:the number of times larger an image appears, compared with the size of the object.
    • Resolution: the clarity of an image; the higher the resolution, the clearer the image.
    • Optical microscopes positives;  -relatively cheap      -easy to use -portable -able to study whole living specimens
    • Optical microscopes negatives: -structures closer than 200nm appear as one object -ribosomes are too small to be examined through an optical microscope
    • Calculating magnification: total magnification = magnifying power of the objective lens x magnifying power of the eyepiece lens
    • Photo-micrograph: a photograph of an image seen through an optical microscope
    • Laser scanning microscopes: -they use laser light to scan an object and assemble the pixel information into a digital image on the computer  -the images produced are high resolution and contrast
      • -these microscopes have depth selectivity and can clearly observe whole living specimens, as well as cells         -they are used in medical profession and biological research
    • Electron microscopes: -use a beam of fast travelling electrons with a wavelength of about 0.004nm which means they have a greater resolution than optical microscopes
      • Transmission electron microscopes: -the specimen is chemically fixed by being dehydrated and stained -the beam of electrons passes through the specimen, which is stained with metal salts
        • -some electrons pass through -the electrons form a 2D black and white image called an electron micrograph -it can produce a magnification of up to 2 million times
      • Scanning electron microscopes: -electrons do not pass through the specimen which is whole but secondary electrons 'bounce off' the specimens surface       -the specimen has to be placed in a vacuum and is coated with  a fine film of metal
        • -it produces a 3D image with a magnification from x15 up to x20,000     -the image is black and white but computer software can be used to add colour
    • Negatives of both types of electron microscopes: -large and very expensive -need lots of skill and training to use -specimens have to be dead as they are viewed whilst in a vacuum -metallic salt stains used for staining specimens may be potentially hazardous to the user
    • The Logarithmic scale: The relative sizes of some biological structures are measured on a logarithmic scale. It shows whether they can/should be viewed using an electron microscope, optical microscope or the human eye


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