Every 9700 Paper 3 has a microscope question, and the graticule arithmetic trips up candidates every session. By hand, carry the stage micrometer onto the stage, choose an objective, turn the focus knob until the scale is sharp and line it up with the eyepiece graticule to calibrate it. Then swap to the leaf epidermis, measure five stomata end to end along the scale, drop to the ×10 objective and count the stomata in one field of view. Finish with your own calculations: actual lengths, field area, stomatal density and the magnification of a drawing. Every attempt uses a different microscope and leaf. Allow about 35 minutes.
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A span of at least ten stage divisions lined up against the eyepiece units at ×40, the length of one eyepiece unit worked out from your own two readings (divisions × 10 µm ÷ units) to 2 or 3 significant figures, and a value that agrees with the microscope. The stage micrometer calibrates the graticule, never the other way round (9700 June 2025 examiner report).
Five lengths in eyepiece units that match stomata on the slide at ×40, each converted with your own calibration, a mean to 2 or 3 significant figures, and the procedure that makes the comparison valid: the same axis of every stoma at the same magnification (June 2024 report).
The field diameter from the stage divisions across the full eyepiece scale at ×10, the area from πr² in mm², a count that matches a field you really looked at in focus, and density = count ÷ area to 2 or 3 significant figures.
The high-power drawing rules (sharp continuous lines, no shading, proportions, ruled label lines), magnification = image ÷ actual with both in the same units and no unit on the answer, and only observable differences between guard cells and the other epidermal cells.
Why one calibration belongs to one objective, and how several random fields and a mean make the density estimate more reliable.
The marking follows the conventions in published Cambridge mark schemes. The experiments, readings and wording on this page are our own.