Core practical: using a light microscope

Core practical 1.6 asks you to prepare a slide, focus a light microscope, draw what you see and work out sizes. You also need to handle very small units, down to the picometre.

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Very small units

Cells and the parts inside them are measured in tiny units. Each unit is 1000 times smaller than the one before.
1 mm = 1000 µm, 1 µm = 1000 nm, 1 nm = 1000 pm
In standard form: 1 mm = 1 × 10-3 m, 1 µm = 1 × 10-6 m, 1 nm = 1 × 10-9 m, 1 pm = 1 × 10-12 m.
PrefixSymbolMeaningExample
millim× 10-3a large plant cell is about 0.1 mm
microµ× 10-6a red blood cell is about 7 µm across
nanon× 10-9a ribosome is about 20 to 30 nm across
picop× 10-12an atom is about 100 pm across

Preparing and viewing a slide

1. Peel a thin layer of onion epidermis (or scrape cheek cells) and lay it flat on a clean slide.
2. Add a drop of stain: iodine solution for onion cells, methylene blue for cheek cells. The stain makes the nucleus and cell walls show up.
3. Lower a coverslip at an angle, using a mounted needle, so that no air bubbles are trapped.
4. Clip the slide on the stage. Start with the lowest-power objective lens, focus with the coarse focusing knob, then the fine knob.
5. Switch to a higher-power lens and refocus with the fine knob only.

Drawing and estimating size

Drawing rules: use a sharp pencil, draw clear unbroken lines, do not shade, label with straight ruled lines, and add a title and the magnification.
Estimating size: if you know the diameter of the field of view, count how many cells fit across it.
size of one cell = field of view diameter ÷ number of cells across
Real size = image size ÷ magnification. Put both sizes in the same unit first.
Worked example

An onion cell measures 30 mm on a drawing made at ×400. What is its real length in micrometres?

  1. Real size = image size ÷ magnification = 30 ÷ 400 = 0.075 mm
  2. Convert to µm: 0.075 × 1000 = 75 µm

Answer: 75 µm

Key idea

1 mm = 1000 µm = 1 000 000 nm. Stain the cells, lower the coverslip at an angle, focus on low power first, and draw in pencil with no shading. Real size = image size ÷ magnification.

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Extra practice

2 more questions

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  1. Convert 2.5 µm into nanometres.

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    Answer: 2500 nm

    1 µm = 1000 nm, so 2.5 × 1000 = 2500 nm

  2. The field of view of a microscope is 1.5 mm across. 5 cells fit across it in a line. Estimate the width of one cell in micrometres.

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    Answer: 300 µm

    1.5 mm = 1500 µm. 1500 ÷ 5 = 300 µm

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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