Use a microscope and scale
Prepare and focus a specimen, make calibrated measurements and estimate cell numbers using a defined sample area.
Cambridge International Biology 9700, 2028–2030. Paper 3 assesses practical skills in a supervised laboratory; these lessons support preparation but do not replace safe, supervised laboratory experience.
Before you begin
- Use biological vocabulary and basic calculations from the AS Biology course. Follow your school laboratory rules and teacher instructions for all practical work.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What must be done before using eyepiece-graticule divisions as an actual length?
What you will learn
- Describe safe, clear microscope setup and slide preparation.
- Use scale bars, magnification and graticule calibration to calculate actual size.
- Estimate cell numbers using fields of view or a grid.
Find and observe the specimen
Prepare a temporary slide as instructed, lower the coverslip carefully, and start with the low-power objective. Centre the feature, bring it into focus with the appropriate focus control, then increase magnification and refocus.
When unfamiliar materials or techniques are used, follow the supplied procedure and the teacher’s safety instructions. Describe only structures that can actually be seen.
Measure with a calibrated scale
For a photomicrograph, actual size = image size ÷ magnification, provided both sizes use the same unit. If a scale bar is shown, use the image-to-bar ratio. For an eyepiece graticule, first calibrate divisions against a stage micrometer at the same objective lens.
Estimate a population in a defined area by counting representative fields or grid squares, recording the sampling method and scaling only when the sampled area supports that estimate.
Pause and explain
A 24 mm image at ×800 represents what actual size?
Worked example
A cell measures 18 mm on a photomicrograph at ×600. Calculate its actual length in micrometres.
Show the worked solution
- Convert 18 mm to 18 000 µm.
- Divide image size by magnification: 18 000 ÷ 600.
- Include the correct unit.
Answer Actual length = 18 000 µm ÷ 600 = 30 µm.
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.
A scale bar representing 40 µm measures 16 mm in the image. A cell measures 10 mm. Find the cell length.
Give me a hint
Use the ratio of the cell image to the scale-bar image.
Compare my reasoning
- One millimetre of image represents 40 ÷ 16 = 2.5 µm.
- Multiply 10 mm by 2.5 µm per mm.
Cell length = (10 ÷ 16) × 40 = 25 µm.
Look for these in your work
- Scale-bar ratio set up
- Actual size and unit correct
At one objective, 50 graticule divisions align with 0.25 mm on a stage micrometer. Find the actual length per division.
Give me a hint
Convert millimetres to micrometres, then divide by the number of divisions.
Compare my reasoning
- 0.25 mm = 250 µm.
- 250 µm ÷ 50 divisions = 5 µm per division.
Each division represents 5 µm at that objective.
Look for these in your work
- Unit conversion correct
- Objective-specific calibration noted
A student estimates cell density from microscope fields. What should they report so the estimate can be interpreted?
Give me a hint
State how and where the sample was counted.
Compare my reasoning
- Describe the field or grid area and the number of samples.
- Report counts and the calculation used to scale to the stated area.
- Avoid generalising beyond the sampled region without evidence.
Report the sampled area, number and placement of fields or grid squares, observed counts, calculation and limits of the estimate.
Look for these in your work
- Sampling area stated
- Calculation and limitation explained
Common mistakes
- Using a graticule calibration from a different objective lens.
- Dividing by magnification before converting units consistently.
- Reporting an estimate without stating its sampled area or method.
What can you explain now?
What must be done before using eyepiece-graticule divisions as an actual length?
Compare with the explanation
Calibrate them against a stage micrometer at that objective
The scale changes when the objective lens changes.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Why must eyepiece-graticule calibration be repeated for a different objective lens?
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.