Resolution & Magnification (CIE A Level Biology)

Magnification

• Magnification is the number of times that a real-life specimen has been enlarged to give a larger view/image
  • E.g. a magnification of x100 means that a specimen has been enlarged 100 times to give the image shown
• A light microscope has two types of lens which allow it to achieve different levels of magnification:
  • An eyepiece lens, which often has a magnification of x10
  • A series of objective lenses, each with a different magnification, e.g. x4, x10, x40 and x100
• To calculate the total magnification of a specimen being viewed, the magnification of the eyepiece lens and the objective lens are multiplied together:

total magnification = eyepiece lens magnification x objective lens magnification

Resolution

• The resolution of a microscope is its ability to distinguish two separate points on an image as separate objects; this determines the ability of a microscope to show detail
  • If resolution is too low then two separate objects will be observed as one point, and an image will appear blurry, or an object will not be visible at all
  • The resolution of a microscope limits the magnification that it can usefully achieve; there is no point in increasing the magnification to a higher level if the resolution is poor
• The resolution of a light microscope is limited by the wavelength of light
  • Visible light falls within a set range of light wavelengths; 400-700 nm
  • The resolution of a light microscope cannot be smaller than half the wavelength of visible light
    • The shortest wavelength of visible light is 400 nm, so the maximum resolution of a light microscope is 200 nm
  • E.g. the structure of a phospholipid bilayer cannot be observed under a light microscope due to low resolution:
    • The width of the phospholipid bilayer is about 10 nm
    • The maximum resolution of a light microscope is 200 nm, so any points that are separated by a distance of less than 200 nm, such as the 10 nm phospholipid bilayer, cannot be resolved by a light microscope and therefore will not be distinguishable as separate objects
• Electron microscopes have a much higher resolution, and therefore magnification, than light microscopes as electrons have a much smaller wavelength than visible light
  • Electron microscopes can achieve a resolution of 0.5 nm

Resolution of light and electron microscopes diagram

The resolving power of electron microscopes is much greater than that of light microscopes due to the smaller wavelength of electrons in comparison to visible light

Comparison of light and electron 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
    • Transmission electron microscopes (TEM) fire electrons through a specimen
    • Scanning electron microscopes (SEM) bounce electrons off the surface of a specimen
  • The electrons are picked up by an electromagnetic lens which then shows the image
  • Electron microscopy requires the specimen to be dead, meaning that they can only be used to capture a snapshot in time, and not active life processes as they occur
  • Electron microscopes are useful for looking at organelles, viruses and DNA as well as looking at whole cells in more detail

Comparing light & electron microscopes table

Electron microscope	Light microscope
Large machines that are permanently installed in laboratories	Small and portable
Need to create a vacuum for electrons to travel through	No vacuum required
Specimen preparation is complex	Specimen preparation can be simple
Maximum magnification of x500 000	Maximum magnification of x2000
Maximum resolution of 0.5 nm	Maximum resolution of 200 nm
Specimens are always dead	Specimens can be living or dead