Science 7–10 · Year 8

Measuring with a microscope: field of view, magnification and a scale bar

Science understanding: Biological sciences

Practical, model not builtLow risk

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The idea

The width of the field of view is a known length at each objective, so the size of a cell can be estimated by counting how many fit across it.

What you need

Compound microscope with a 10x eyepiece marked with its field number (for example WF10x/18) and 4x, 10x and 40x objectives
A clear plastic ruler with millimetre marks, or a stage micrometer
Prepared onion epidermis slide (from the plant and animal cells practical) and a prepared slide of a known object
Calculator and a results table

How to do it

Place the ruler on the stage and focus at 4x. Count the millimetre marks across the widest part of the field and record the field diameter.
Calculate the field at 10x and 40x by dividing the 4x field by 2.5 and 10 (the ratio of objective magnifications). Check the 10x value by direct measurement if the marks are visible.
Calculate total magnification for each objective: eyepiece 10x multiplied by objective.
Focus on the onion slide at 100x. Count how many cells lie end to end across the field; cell length = field diameter divided by the number of cells.
Repeat the count on three different fields and average.
Draw one cell and add a scale bar whose length is calculated from the field diameter.

What you should see

With a field number of 18 the field is 4.5 mm at 4x, 1.8 mm at 10x and 0.45 mm at 40x, and total magnifications are 40x, 100x and 400x; a field number of 20 gives 5.0, 2.0 and 0.50 mm. The count of cells across the field gives the learner's own cell length: six cells spanning the 1.8 mm field means each is about 0.3 mm long. The SAPS onion growth resource reports no value here; it sets the question for learners, asking whether the cells change in length, in width or in both as the epidermis grows, so learners who peel different layers of the bulb should expect different values and should record which layer they used. The learner knows the method worked when the 10x field measured directly agrees with the value calculated from 4x.

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.

Higher magnification shows a larger area.
A magnification label such as x400 stays correct when a drawing or photograph is enlarged; it does not, but a scale bar stays correct because it is enlarged with the image.
Total magnification is the objective alone.

Safety card

Low riskLearners carry it out

Hazards

Glass slides

Controls

Carry the microscope with two hands; report breakages

Note

No hazardous chemicals; record a RiskAssess risk assessment for the activity as school procedure requires.

Curriculum references

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

Science 7–10 Syllabus (2023)SC4-CLS-01 SC4-WS-01 SC4-WS-05
Australian Curriculum v9AC9S8U01AC9S8I03AC9S8I04

Sources

The pages the author read to write this activity.

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