Magnification is image size divided by actual size, and both must be in the same unit before you divide. Most lost marks come from skipping that conversion.
This lesson is part of cells and organisation. To see what these sizes mean, read relating organelles to functions.
What is the unit-first method?
Convert first, then divide. Write both sizes in micrometres, because cells are measured in µm and a ruler gives mm.
- Measure the image in mm.
- Multiply by 1 000 to change mm to µm.
- Divide image size by actual size.
- Write the answer as ×number.
Worked example 1: find the magnification
A drawing of a cheek cell is 45 mm long. The actual cell is 30 µm long.
Convert: 45 mm × 1 000 = 45 000 µm.
Magnification = 45 000 ÷ 30 = ×1 500.
Worked example 2: find the actual size
A micrograph shows a leaf cell 60 mm wide at ×400. Find the actual width in µm.
Actual size = image size ÷ magnification. Convert first: 60 mm = 60 000 µm.
Actual size = 60 000 ÷ 400 = 150 µm.
Worked example 3: find the image size
A student wants a drawing of a 25 µm cell at ×800. How long should the drawing be in mm?
Image size = magnification × actual size = 800 × 25 = 20 000 µm.
Convert back: 20 000 ÷ 1 000 = 20 mm.
The mistake that costs marks
The slip is dividing 45 by 30 and writing ×1.5. The numbers look tidy, so the error feels invisible.
Sense-check instead. A cell 30 µm wide is tiny, and a drawing 45 mm long is huge by comparison, so the magnification must be in the thousands, not close to 1. If the answer looks too small, you forgot to convert.
| Step | Wrong | Right |
|---|---|---|
| Units | 45 mm and 30 µm | 45 000 µm and 30 µm |
| Division | 45 ÷ 30 | 45 000 ÷ 30 |
| Answer | ×1.5 | ×1 500 |
Check yourself
A photograph of an onion cell is 32 mm long. The actual cell is 160 µm long. Calculate the magnification.
Answer
Convert the image: 32 mm × 1 000 = 32 000 µm.
Magnification = 32 000 ÷ 160 = ×200.
Sense-check: a 160 µm cell drawn at 32 mm is about 200 times larger, which fits.
What to study next
Practise all three question types in the cells and organisation practice set, and use the units and significant figure checker to confirm your conversions. If you would like a teacher to watch your working, see online one-to-one Biology tuition.