2.2 Size of specimens
- Syllabus
- 0610–2026–2027
- Topic
- 2.2
- Level
- —
Learning objectives
2.2.1Magnification formula• State and use the formula: magnification = image size ÷ actual size2.2.2Magnification and size of biological• Calculate magnification and size of biological specimens using millimetres as units2.2.3mm and micrometre conversions• Convert measurements between millimetres (mm) and micrometres (μm)
Use the magnification relationship
magnification = image size ÷ actual size
| Unknown | Rearranged relationship |
|---|---|
| magnification | image size ÷ actual size |
| image size | magnification × actual size |
| actual size | image size ÷ magnification |
Convert image size and actual size to the same unit before dividing. Magnification is a ratio, so it has no unit and is written, for example, ×500.
If an image is 45 mm long and the specimen is 25 mm long, magnification = 45 ÷ 25 = ×1.8.
Do not divide values with different units. A millimetre image divided directly by a micrometre actual size gives a factor 1000 error.
Calculate specimen and image sizes in millimetres
Measure the requested image dimension, write all lengths in millimetres, choose the correct rearrangement, substitute values and include the unit on a size answer.
| Step | Check |
|---|---|
| 1 | measure exactly between the labelled endpoints |
| 2 | convert both lengths to mm if needed |
| 3 | select M = I ÷ A, I = M × A, or A = I ÷ M |
| 4 | calculate and sense-check the result |
An image is 5.00 mm long at ×250. Actual size = 5.00 ÷ 250 = 0.0200 mm. Check: 0.0200 mm × 250 = 5.00 mm.
If a printed scale bar represents a stated real length, its measured image length can be used to find the magnification; the same magnification then applies to the specimen on that image.
Use the dimension named in the question, not the whole page or an unlabelled maximum width. Magnification is unitless; image and actual sizes carry mm.
Convert between millimetres and micrometres
1 mm = 1000 μm
| Direction | Operation | Example |
|---|---|---|
| mm → μm | multiply by 1000 | 0.0315 mm = 31.5 μm |
| μm → mm | divide by 1000 | 65 μm = 0.065 mm |
Multiplying by 1000 moves the decimal three places right; dividing by 1000 moves it three places left. Equivalently, 1 μm = 0.001 mm = 10⁻³ mm.
1.276 μm ÷ 1000 = 0.001276 mm. Reversing the conversion, 0.001276 mm × 1000 returns 1.276 μm.
Micrometres are smaller than millimetres, so the numerical value must become larger when converting mm to μm and smaller when converting μm to mm.