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Check your answer after each question. Use a calculator where needed.
Learn the method
How to approach these questions
- Write down the image size and actual size, including their units.
- Convert to the same unit: 1 mm = 1,000 μm.
- Rearrange if needed: actual size = image size ÷ magnification; image size = actual size × magnification.
- For a scale bar, compare the measured object length with the measured bar length. Multiply that ratio by the real length represented by the bar.
Worked examples
A printed micrograph shows a bacterium 27.0 mm long. Its actual length is 4.50 µm. Calculate the magnification.
Convert 27.0 mm to 27 000 µm. Magnification = 27 000 ÷ 4.50 = ×6000. Magnification is a ratio with no unit; 6.00 × 10³ expresses the same value to three significant figures.
On the same micrograph, a cell measures 31.5 mm and a scale bar measures 7.00 mm. The bar represents 2.00 µm. Find the actual cell length.
Cell-to-bar ratio = 31.5 ÷ 7.00 = 4.50. Actual cell length = 4.50 × 2.00 = 9.00 µm. Resizing the whole image changes both measured lengths by the same factor, so the ratio is preserved. Use the supplied measurements; measuring this webpage with a ruler would depend on screen settings.
A cell is 8.00 µm wide and is shown at ×2500. Calculate its image width in mm.
Image width = 8.00 µm × 2500 = 20 000 µm = 20.0 mm. Multiplying by magnification gives an image length, then dividing by 1000 converts µm to mm.
Keep practising
Topic map and study list · Practical data guides · Active recall decks
Course resources
AQA A-level Biology · OCR A A-level Biology · Textbook answers
Skill coverage follows the common A-level science subject content. Check your board’s current requirements: AQA 7402 and OCR A H420.