Microscopes – Biology AS A Level – Revision notes

In Biology AS, you will need to know about 3 types of microscopes: light microscopes, scanning electron microscopes and transmission electron microscopes.

Most of the time, the exam paper will have questions asking about the advantages and disadvantages of using these microscopes but in order to understand this, it is best that you first have a brief overview of how each type of microscope works.

How a light microscope works

  • A condenser is used to gather light from a source and focus it on a small area of the thin specimen.
  • The specimen is illuminated by the light shone from beneath it.
  • The objective lens are small and have a shorter focal length which allows light to be gathered from the illuminated specimen and brings the image into focus at a shorter distance.
  • The ocular lens magnify the image as it is brought to your eyes.
  • The objective lenses can be used to bring even smaller areas of the specimen into view.

How scanning electron microscopes (SEM) works

  • An SEM is a microscope that uses electrons to allow you to see objects smaller than the wavelength of light
  • The electron gun at the top of the column generates a beam of electrons that is fired at the sample surface (the lenses help to direct the electrons onto the target).
  • The surface of the sample is scanned with a high energy beam of electrons which causes the electrons to collide with the atoms in the sample
  • The detectors pick up on the movement of the secondary electrons and it is then fed through to the sensor/monitor that collects the image.

How transmission electron microscopes (TEM) work

  • The TEM also works by using a beam of electrons to create an image of the specimen but, unlike the SEM that produces an image by bouncing electrons off the surface of the sample, the TEM fires the electrons through the sample.
  • The magnetic lenses form a high voltage electron beam that is emitted by the cathode and shot through the sample
  • The electromagnetic lenses help to control the path of electrons to hit a fluorescent screen and produce an image.


*note – the diagrams do not belong to me*


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