A drawing or photograph of a cell is far bigger than the cell itself. One formula links the image size, the actual size and the magnification, and it rearranges to find any one of them. Converting between millimetres and micrometres is Extended content.
magnification = image size ÷ actual size. Rearranged, actual size = image size ÷ magnification, and image size = magnification × actual size. Measure the image with a ruler in millimetres and put both sizes in the same unit before dividing. Magnification has no unit; it is written as ×600, for example. Extended: 1 mm = 1000 μm, so multiply by 1000 to go from mm to μm and divide by 1000 to go from μm to mm.
Choose find magnification. Drag the ruler so its zero sits on line A, read the length at line B, then divide by the actual size given. The ruler is numbered in centimetres with millimetre marks, so 3.6 on the ruler is 36 mm.
Measure the image in millimetres, not centimetres. A drawing 3.6 cm long is 36 mm. Using 3.6 with an actual size in mm gives a magnification ten times too small.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Which formula is used to calculate magnification?
A pollen grain is 0.05 mm across. Its image is 50 mm across. What is the magnification?
What is the unit of magnification?
A student measures an image as 3.6 cm long. What length should she use in a magnification calculation in millimetres?
Switch the simulator to find actual size and find image size. The same three quantities appear every time; only the unknown changes.
| Specimen | Known | Working | Answer |
|---|---|---|---|
| flea | image 50 mm, ×20 | actual = 50 ÷ 20 | 2.5 mm |
| onion epidermis cell | actual 0.25 mm, ×200 | image = 200 × 0.25 | 50 mm |
| pollen grain | image 50 mm, actual 0.05 mm | magnification = 50 ÷ 0.05 | ×1000 |
| Amoeba | image 54 mm, ×120 | actual = 54 ÷ 120 | 0.45 mm |
Extended content. Cells are so small that their sizes are usually given in micrometres (μm). Drag the zoom slider from a bacterium up to a flea and read each size in both units.
Never divide a size in mm by a size in μm. Dividing 40 mm by 8 μm as if they were the same unit gives ×5, a thousand times too small. Convert both sizes to the same unit first.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
A drawing of a flea is 50 mm long. The magnification is ×20. What is the actual length of the flea?
An onion epidermis cell is 0.25 mm long. It is drawn at a magnification of ×200. How long is the drawing?
Extended: what is 0.045 mm in micrometres?
Extended: a red blood cell is 8 μm in diameter. In a photograph its image is 40 mm across. What is the magnification?
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