Edexcel International A Level Biology

Revision Notes

3.7 Microscopy: Magnification & Resolution

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Magnification & Resolution


  • Magnification is how many times bigger the image of a specimen observed is in comparison to the actual, real-life size of the specimen
  • A light microscope has two types of lens
    • An eyepiece lens which often has a magnification of x10
    • A series of, usually 3, objective lenses, each with a different magnification
  • To calculate the total magnification, the magnification of the eyepiece lens and the objective lens are multiplied together

total magnification = eyepiece lens magnification x objective lens magnification


  • Resolution, or resolving power, is the ability to distinguish between two separate points
    • If two separate points cannot be resolved, they will be observed as one point and the image will be unclear
  • The resolution of a microscope limits the magnification that it is capable of; there is no point in magnifying an image at low resolution as this will just result in a big blur rather than a small blur!
  • The resolution of a light microscope is limited by the wavelength of light; the wavelength of light is too long to allow for high resolution
    • E.g. the phospholipid bilayer structure of the cell membrane cannot be observed under a light microscope
      • The width of the phospholipid bilayer is about 10 nm
      • The maximum resolution of a light microscope is 200 nm 
      • Any points that are separated by a distance less than 200 nm cannot be resolved by a light microscope and therefore will not be distinguishable as separate points on an image
  • Electron microscopes have a much higher resolution, and therefore magnification, than a light microscope as electrons have a much smaller wavelength than visible light

Comparing resolution, downloadable AS & A Level Biology revision notes

The resolving power of an electron microscope is much greater than that of the light microscope due to the smaller wavelength of electrons

Comparing electron & light microscopes

  • Light microscopes are used for specimens larger than 200 nm
    • Light microscopes shine light through the specimen
    • The specimens can be living, and therefore can be moving, or dead
    • Light microscopes are useful for looking at whole cells, small plant and animal organisms, and tissues within organs such as in leaves or skin
  • Electron microscopes, both scanning and transmission, are used for specimens larger than 0.5 nm
    • Electron microscopes fire a beam of electrons at the specimen
    • The electrons are picked up by an electromagnetic lens which then shows the image
    • Electron microscopy requires the specimen to be dead; this can provide a snapshot in time of what is occurring in a cell, e.g. DNA can be seen replicating and chromosome position within the stages of mitosis are visible
    • Electron microscopes are useful for looking at organelles, viruses, and DNA, as well as looking at whole cells in more detail

Light v Electron Microscope TableComparison of the Electron Microscope and Light Microscope, downloadable AS & A Level Biology revision notes

Staining Specimens

  • Specimens to be viewed under a microscope sometimes need to be stained, as the cytoplasm and other cell structures may be transparent or difficult to distinguish
    • Note that most of the colours seen in images taken using a light microscope are the result of added stains
      • Chloroplasts are the exception to this; they show up green, which is their natural colour
  • The type of stain used is dependent on what type of specimen is being prepared and what the researcher wants to observe within the specimen
    • Different molecules absorb different dyes depending on their chemical nature
  • Specimens or sections are sometimes stained with multiple dyes to ensure that several different tissues within the specimen show up; this is known as differential staining
  • Some common stains include
    • Haemotoxylin
      • Stains plant and animal cell nuclei purple, brown or blue
    • Methylene blue
      • Stains animal cell nuclei blue
    • Acetocarmine
      • Stains chromosomes in dividing nuclei of plant and animal cells
    • Iodine
      • Stains starch-containing material in plant cells blue-black
    • Toluidine blue 
      • Stains tissues that contain DNA and RNA blue
    • Phloroglucinol
      • Stains a chemical called lignin found in some plant cells red/pink

Leaf Photomicrograph

Toluidine blue and phloroglucinol have been used to stain this tissue specimen taken from a leaf

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Author: Alistair

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.