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AS · Practice questions · Microscopes, magnification and resolution

Same unit, then divide.

Six original Cambridge-style questions on the microscope in cell studies: magnification, scale bars, eyepiece graticules and resolution.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Magnification is size; resolution is detail.

01
Recall
[2 marks]

Define magnification and resolution.

  • Magnification: the number of times larger an image is than the actual object (image size ÷ actual size). ✓
  • Resolution: the smallest distance between two points that can be distinguished as separate points. ✓
02
Calculation
[2 marks]

A chloroplast measures 42 mm across in an electron micrograph at ×8000. Calculate its actual diameter in μm.

  • 42 mm = 42 000 μm. ✓
  • Actual size = 42 000 ÷ 8000 = 5.25 μm (5.3 μm). ✓
03
Calculation
[3 marks]

A scale bar labelled 2 μm is 30 mm long. A bacterium in the same micrograph is 45 mm long. Calculate the magnification and the actual length of the bacterium.

  • 30 mm = 30 000 μm; magnification = 30 000 ÷ 2 = ×15 000. ✓
  • Bacterium image = 45 000 μm. ✓
  • Actual length = 45 000 ÷ 15 000 = 3.0 μm. ✓
04
Calculation
[3 marks]

A student lines up an eyepiece graticule with a stage micrometer (1 small division = 10 μm) using the ×10 objective. 50 graticule divisions match 90 micrometer divisions. A root hair cell is then 23 graticule divisions long. Calculate its length.

  • 90 micrometer divisions = 900 μm. ✓
  • 1 graticule division = 900 ÷ 50 = 18 μm. ✓
  • Length = 23 × 18 = 414 μm (0.414 mm). ✓
05
Explain
[3 marks]

Explain why a transmission electron microscope can show the cristae of a mitochondrion but a light microscope cannot.

  • Cristae are separated by distances far less than 200 nm. ✓
  • Light has a wavelength of 400 to 700 nm, so the light microscope cannot resolve points closer than about 200 nm. ✓
  • Electrons have a much shorter wavelength, so the TEM resolves to about 0.5 nm and the cristae appear as separate structures. ✓
06
Analysis
[2 marks]

A student wants to add a 20 μm scale bar to a drawing of a cell made at ×600. Calculate how long the bar should be, and state one difference between a TEM and an SEM image.

  • Bar length = 20 × 600 = 12 000 μm = 12 mm. ✓
  • A TEM image is a 2D section through the specimen; an SEM image shows the 3D surface (TEM also has the higher resolution). ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.