Microscopy Virtual Lab: Calibrate a Graticule, Measure Cells

Updated 2026-10-07

This microscopy virtual lab follows the AQA GCSE Biology required practical on using a light microscope, and the graticule calibration that A-level and IB students do before they measure cells. Students make a temporary mount of onion epidermis, focus from low to high power, calibrate the eyepiece graticule against a stage micrometer for each objective, and use the calibration to measure a cell in micrometers. One period gives a calibration table, a measured cell and a clear reason why the graticule must be calibrated.

Microscopy lab – slides, calibration, cell size and electron microscopes

Curriculum links

AQA GCSE Biology 4.1.1.5 (microscopy) and its required practical: use a light microscope to observe, draw and label plant and animal cells, with magnification = image size ÷ actual size.
AQA A-level Biology 3.2.1.3 (methods of studying cells): calibrating an eyepiece graticule with a stage micrometer, and the resolution of light and electron microscopes.
IB Biology A2.2.2 (microscopy skills) and NGSS MS-LS1-1 (living things are made of cells).

Simulic is not affiliated with or endorsed by AQA, the IB or any exam board.

Before the lab (5 min)

Ask students to commit to a prediction, on paper or as question 1 of the class link:

"At ×10, one eyepiece division covers 10.4 µm of the slide. You turn the ×40 objective into place. How much of the slide does one eyepiece division cover now?"

Many students say "the same, the scale hasn't moved". That is the point of the lab: the graticule sits in the eyepiece, so it looks identical at every power, but the specimen under it is magnified more.

Method in the simulation

The sim opens on the Microscope tab with onion epidermis. Calibrate first, then make the onion slide, because changing the sample starts a new slide.

Stage micrometer. Set Sample to Stage micrometer and press Put on the stage. Raise the stage with the coarse focus knob until the image is sharp at ×4 (the readout says "The image is in focus").
Calibrate ×4, ×10 and ×40. For each objective, drag the slide until the 0 lines of both scales line up. Find a graticule line that lines up exactly with a micrometer line, as far from 0 as you can (use Magnifier ×4). Enter both numbers and 1 eyepiece division = micrometer divisions × 10 µm ÷ eyepiece divisions, then press Check. At ×40 use only the fine focus knob.
Onion slide. Set Sample to Onion epidermis + iodine. Choose the correct option at each step: iodine, a very thin layer, lower the coverslip at 45°, blot, put on the stage. Focus at ×4, then turn to ×10.
Measure. Turn on the Eyepiece graticule. Without dragging the slide, count the divisions along the cell under the red +. Enter the divisions and the actual length, then press Record. The sim checks both numbers.
Drag to two more cells, measure them, and read the mean under the table.

Objective	Eyepiece divisions	Micrometer divisions	1 eyepiece division (µm)
×4
×10
×40

Cell	Objective	Length (eyepiece divisions)	Actual length (µm)
Under the + (not moved)	×10
2	×10
3	×10

the stage micrometer at ×10 with both 0 lines aligned and the calibration table showing ×10 correct, 10.42 µm

Expected results

All values come from the simulation.

Objective	Calibration that lines up	1 eyepiece division	Field of view
×4	35 eyepiece = 91 micrometer divisions	26.0 µm	about 4,690 µm
×10	48 eyepiece = 50 micrometer divisions (or 96 = 100)	10.4 µm	about 1,880 µm
×40	96 eyepiece = 25 micrometer divisions	2.60 µm	about 470 µm

The values are not exactly 25, 10 and 2.5 µm. The true magnification of each objective is a few percent below its label, which is why the graticule is calibrated for every objective. The sim accepts readings within 3%.
The onion cell under the + is about 17 divisions long at ×10: 17 × 10.4 ≈ 181 µm. At ×40 the same cell spans about 69 divisions (69 × 2.60 ≈ 180 µm).
Other onion cells are 170 to 340 µm long and 53 to 71 µm wide, so a mean of three cells usually lands between 220 and 290 µm.

Questions for students

(Prediction, asked again after the lab) At ×40, how much of the slide does one eyepiece division cover?
During calibration, which length is fixed and known before you start?
Calibrate the graticule for the ×40 objective. What is one eyepiece division in µm?
Without moving the slide, measure the onion cell under the + at ×10. What is its actual length?
A partner skips calibration and assumes one division is 2.5 µm at ×40. Explain the problem.

Answers for teachers: (1) About 2.6 µm, a quarter of the ×10 value. (2) One stage micrometer division, 10 µm. (3) 2.60 µm (accept 2.52 to 2.68). (4) About 181 µm (accept 169 to 193). (5) The objective's true magnification differs from its label, so 2.5 µm is about 4% too small. The cell would come out as 69 × 2.5 ≈ 173 µm instead of 180 µm: a systematic error in every measurement.

Common misconceptions

"The graticule scale is in µm." Its divisions have no fixed size. They only mean something after calibration, and the value changes with the objective.
"Higher magnification always shows more detail." Resolution limits detail. The Light vs electron tab shows ribosomes (about 25 nm) staying invisible in a light microscope however far you zoom.
"Black-rimmed circles are cells." They are air bubbles trapped by dropping the coverslip flat. Try it on purpose in the sim.

Extension

Magnification tab: practice M = I ÷ A with mm to µm conversions. The sim flags answers that are out by a factor of 1000.
Light vs electron tab: compare the resolution of the light microscope (about 200 nm), SEM and TEM, and explain why only the TEM shows the membrane bilayer.

FAQ

Does this replace the required practical?

No. AQA required practicals must be done by hand. Use the simulation to teach calibration before the wet lab, or for students who missed it. See virtual labs vs physical labs.

Why does my group get 10.4 µm and not 10 µm?

Each objective's real magnification is a little below its label in this model, as with real lenses. That small difference is exactly what calibration corrects.

Can students skip making the slide?

Yes. In the starting values on your class link, tick "Slide already made, on the stage and in focus". Keep the slide-making steps if you want students to see the effect of each mistake.