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Microscopes

Biology: AQA A Level, topic 2.1.7

Microscopes

It is important to understand the principles and limitations of optical microscopes, transmission electron microscopes and scanning electron microscopes.

Optical (light) microscopes

Optical (light) microscopes

  • Visible light passes and is bent through the lens system to enable the user to see the specimen.
  • The specimen can be alive.
  • Individual cells are generally transparent and their components are not distinguishable unless they are coloured with special stains.
  • Staining usually kills the cells.
Uses of light microscopes

Uses of light microscopes

  • Most student microscopes are classified as light microscopes.
  • Maximum resolution is 0.2 micrometres.
  • The nucleus and mitochondria can be seen with a light microscope.
  • The maximum magnification is around x1,500.
Electron microscopes

Electron microscopes

  • In contrast to light microscopes, electron microscopes use a beam of electrons instead of a beam of light.
  • This allows higher magnification and higher resolving power.
  • This means that more detail can be seen.
Uses of electron microscopes

Uses of electron microscopes

  • Electron microscopes have a maximum resolution of 0.0002 micrometres.
  • This is around 1000 times more than light microscopes.
  • The maximum magnification is around x1,500,000.

Types of Electron Microscopes

There are two main types of electron microscopes: transmission (TEM) and scanning (SEM) electron microscopes.

TEM

TEM

  • In a TEM, the electron beam penetrates the cell and provides details of a cell’s internal structures.
  • TEMs use electromagnets to focus the electron beam.
  • TEMs are high resolution microscopes.
  • In thin specimens, you can see the internal structures of organelles such as chloroplasts.
SEM

SEM

  • In a SEM, a beam of electrons moves back and forth across a cell’s surface, creating details of cell surface characteristics.
  • SEMs knock electrons off the specimen and these electrons come together to form an image.
  • SEM images can be three-dimensional.
  • Specimens do NOT have to be thin like when using a TEM.
  • Resolution is lower than that produced by a TEM.

Test yourself on Microscopes

  • What is the maximum resolution and magnification of an optical microscope?

  • What are the two main types of electron microscopes?

  • Which of the following statements regarding the TEM is true?

  • Which of the following does NOT apply to SEM?

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