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Microscopy: magnification, resolution and what you can trust questions

The difference between magnification and resolution, how wavelength sets the resolving limit, light microscopes against transmission and scanning electron microscopes, the magnification equation with unit conversions, calibrating an eyepiece graticule with a stage micrometer, and artefacts.

6 original questions · 20 marks · the microscopy: magnification, resolution and what you can trust notes · Cells, microscopy and biological organisation

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• 4 marks · Calculate

  An eyepiece graticule is calibrated against a stage micrometer whose divisions are each 10 µm long. 25 eyepiece divisions are found to cover 4 stage micrometer divisions. A cell then measures 15 eyepiece divisions across. Calculate the diameter of that cell in micrometres.

  Mark scheme

  1. M1 4 stage micrometer divisions correspond to 4 × 10 = 40 µm
  2. M1 one eyepiece division is worth 40 divided by 25
  3. M1 = 1.6 µm per eyepiece division, then multiplied by the 15 divisions the cell covers
  4. A1 a final answer of 24 µm

• 4 marks · Explain

  Explain why a transmission electron microscope resolves finer detail than a light microscope, and explain why it cannot be used to look at a living cell.

  Mark scheme

  1. B1 resolution is limited by the wavelength of the radiation used to illuminate the specimen
  2. B1 a beam of electrons has a far shorter wavelength than visible light, so points much closer together can still be seen as separate
  3. B1 the electron beam has to travel through a vacuum, because air molecules deflect electrons and scatter the beam
  4. B1 a vacuum removes the water from a cell, so the specimen must be dead, fixed and dehydrated before it is put in

• 4 marks · Compare

  Compare a transmission electron microscope with a scanning electron microscope, referring to the specimen used, the electrons detected, the image produced and the resolution achieved.

  Mark scheme

  1. B1 a transmission instrument needs an ultrathin section, whereas a scanning instrument takes a whole specimen coated in a thin film of metal
  2. B1 a transmission instrument detects electrons that have passed through the specimen, whereas a scanning instrument detects electrons knocked off its surface
  3. B1 a transmission instrument gives a two-dimensional image of a section, whereas a scanning instrument gives a three-dimensional image of a surface
  4. B1 a transmission instrument resolves to about 0.1 nm, which is better than the few nanometres a scanning instrument manages

• 3 marks · Calculate

  A ribosome measures 5 mm across on an electron micrograph labelled as being at a magnification of 200 000 times. Calculate the actual diameter of the ribosome, giving your answer in nanometres.

• 3 marks · Suggest

  A student examining an electron micrograph counts what appear to be forty separate mitochondria, of widely differing sizes and shapes, in one small cell. Suggest two reasons why this count and these shapes may be misleading.

• 2 marks · State

  State what is meant by the magnification of an image, and state what is meant by the resolution of a microscope.

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