Use an eyepiece graticule and stage micrometer
A stage micrometer calibrates eyepiece divisions so an unknown specimen length can be measured in real units.
AS Level · Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; supervised practical skills and human subject-teacher review remain necessary.
Before you begin
- Know that cells are the basic units of living organisms and that a light microscope uses lenses to form an enlarged image.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What establishes the real length represented by eyepiece-graticule divisions?
What you will learn
- Describe how to calibrate an eyepiece graticule with a stage micrometer
- Use calibrated divisions and report lengths in mm, µm or nm
Calibrate each objective setting
An eyepiece graticule has arbitrary divisions. Align it with the known scale of a stage micrometer and determine how many eyepiece divisions span a known stage length. Divide the known stage length by the number of aligned eyepiece divisions to find the real length per division.
Repeat calibration for each objective lens because the field of view changes with magnification. Record the calibration and objective used. The eyepiece scale alone does not have a fixed length until calibrated.
Measure specimens and convert units
After calibration, align the specimen with the eyepiece graticule and multiply the number of divisions by the calibrated length per division. Use the appropriate unit: millimetres, micrometres or nanometres.
Keep the stage micrometer out of the specimen measurement once calibration is complete. A stage micrometer is the known reference; the calibrated eyepiece graticule is used to measure the specimen.
Pause and explain
Why should the eyepiece graticule be recalibrated after changing objective lens?
Worked example
Twenty eyepiece divisions align with 100 µm on the stage micrometer at one objective. A specimen spans 7 divisions. Find its length.
Show the worked solution
- Find the length represented by one eyepiece division.
- Multiply by the specimen’s divisions.
Answer One division = 100 ÷ 20 = 5 µm. Specimen length = 7 × 5 = 35 µm.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
At one objective, 25 eyepiece divisions equal 0.50 mm on a stage micrometer. Calculate the length per eyepiece division in micrometres.
Give me a hint
Convert 0.50 mm to micrometres, then divide by 25.
Compare my reasoning
- 0.50 mm = 500 µm.
- 500 ÷ 25 = 20 µm per division.
Each eyepiece division represents 20 µm at this objective.
Look for these in your work
- mm converted to µm
- Calibration value correct
A cell spans 9 eyepiece divisions. Calibration at this objective is 4 µm per division. Find the cell length.
Give me a hint
Multiply divisions by the calibrated value.
Compare my reasoning
- 9 × 4 = 36.
- Attach micrometres.
Cell length = 9 × 4 µm = 36 µm.
Look for these in your work
- Multiplication correct
- Unit included
A student records 1.8 mm as 180 nm. Identify and correct the unit conversion.
Give me a hint
Use 1 mm = 1000 µm and 1 µm = 1000 nm.
Compare my reasoning
- 1.8 mm = 1800 µm.
- 1800 µm = 1 800 000 nm.
1.8 mm equals 1800 µm, which equals 1 800 000 nm. The student’s value is too small by a factor of 10 000.
Look for these in your work
- Both conversions shown
- Correct scale stated
Common mistakes
- Assuming one eyepiece division has the same value at every objective
- Measuring a specimen with the stage scale instead of the calibrated eyepiece
- Forgetting to include a length unit
What can you explain now?
What establishes the real length represented by eyepiece-graticule divisions?
Compare with the explanation
Calibration against a stage micrometer
The stage scale has known divisions.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
How do a stage micrometer and an eyepiece graticule work together, and why calibrate each objective?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.