Light Microscopes and Electron Microscopes
Light microscopes were invented in the seventeenth century, and since then they have remained basically the same.
The light microscope works by having a light shining up from a source beneath the stage, the light is transmitted through two glass lenses in series; the objective and ocular (eyepiece) lenses. Depending on the strength of these lenses, the light microscope can routinely magnify an object up to four hundred times.
The main problem with the light microscope is that there is a limit as to the detail it can show due to the resolving power.
Resolving power is the minimum distance by which two separate points must be separated in order that they are perceived as two separate points and not one fused image. In the light microscope this distance is approximately 0.2 um.
The limited resolution of the light microscope is imposed by the wavelength of visible light and means that little can be gained by magnifying objects more than fifteen hundred times as there is a limit upon structural detail that can be depicted within a cell.
Due to the limitations of the light microscope, since the 1950’s, major microscopic studies have been done using the electron microscope, which uses an electron beam as opposed to light, and electromagnets instead of glass lenses. The electrons are recorded on a photographic plate, which forms a viewable image on a screen (however this is only in black and white, unlike the light microscope, which allows objects to be viewed in full colour.)
Electron beams have a much shorter wavelength than light, therefore modern electronic microscopes have a resolving power one thousand times greater than optical microscopes. This means that objects can be magnified to a far greater extent without loss of clarity. The electron microscope can magnify images up to five hundred thousand times. This has meant that many discoveries have been made, for example: materials, which had in the past, been described as structureless were shown to have an elaborate internal organisation.
The main problem with the electron microscope, however, is that material must be mounted in a vacuum, there is therefore, no scope for study of live material. Also, preliminary treatment may distort delicate structures inside cells and create images that are not ‘real’ (these are called artefacts.)
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