Investigating Science 11–12 · Year 11

Measuring cells with a calibrated eyepiece graticule

Module 1: Cause and Effect – Observing

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

A drawing or description of a cell stays qualitative until the microscope is calibrated; a graticule checked against a stage micrometer turns each observation into a measurement with a stated uncertainty.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

iodine solution stains skin and irritates eyes
scalpel cuts
broken slides and coverslips

Controls

safety glasses when handling iodine
cut away from the body on a tile
broken glass into the glass bin

Note

Chemicals are used: follow the NSW Department of Education Chemical Safety in Schools (CSIS) package, Section 1 (general information for all staff, including the CSIS 1.7 risk assessment requirement), read the Safety Data Sheet for each chemical and complete a RiskAssess risk assessment before the lesson.

What you need

light microscope with ×4, ×10 and ×40 objectives and a ×10 eyepiece fitted with a graticule (10 mm scale in 100 divisions)
stage micrometer (1 mm in 100 divisions of 10 µm)
onion bulb, dilute iodine (Lugol’s) solution, slides, coverslips, forceps, scalpel, paper towel
a commercially prepared slide of an animal tissue for comparison

How to do it

Put the stage micrometer on the stage and focus at ×4; line up the scales and count how many graticule divisions match a known length on the micrometer.
Calculate the length of one graticule division in micrometres; repeat at ×10 and ×40.
Peel a piece of onion epidermis, mount it in a drop of iodine solution and add a coverslip.
At ×10, measure the length and width of 10 cells in graticule divisions and convert to micrometres; at ×40 measure 5 nuclei.
Repeat the measurements on the prepared animal tissue slide.
Report each size as a mean and range with an uncertainty of ± half a division, alongside the qualitative description of each cell type.

What you should see

The graticule sits in the image formed by the objective, so each 0.1 mm division spans 100 µm divided by the objective magnification: 25 µm at ×4, 10 µm at ×10 and 2.5 µm at ×40. The stage micrometer confirms or corrects these values, because objective magnifications are nominal, and the corrected factor is the one the learner uses. Onion epidermal cells appear as elongated brick-shaped cells with a stained nucleus and no chloroplasts; their measured lengths and widths are reported as mean ± range with ± 5 µm reading uncertainty at ×10, and the learner compares their value with a partner’s measurement of the same cell.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Total magnification alone tells you how big the cell is.
Higher magnification always gives a more accurate measurement.
All plant cells are green because they contain chloroplasts.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Investigating Science Stage 6 Syllabus (2017), NESA; currentINS11-8 INS11/12-1 INS11/12-3 INS11/12-4
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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