2.1.4Organisation of the Organism
Converting Between Units — Cambridge IGCSE Co-ordinated Sciences 0654 - Biology Revision Notes & Topical Questions
5 min read 6 parts Lesson 5 of 5
Part 1 of 4Why conversion is needed (Extended)
Two different scales
- Two different scales The real sizes of cells and organelles are usually quoted in micrometres (µm), because they are so small.
- A ruler measures the image in millimetres (mm), which is a much larger unit.
Units must match
- Units must match The magnification equation only works when the image size and actual size are in the same unit.
- Whenever a question mixes mm and µm, you must convert one of them before you calculate anything.
The biggest source of lost marks
- The biggest source of lost marks Forgetting this conversion is the most common error in magnification questions, so make it an automatic first step.
Part 2 of 4The key relationship
| Conversion | Operation | Example |
|---|---|---|
| 1 mm = 1000 µm | This is the fact to learn | A 0.5 mm structure is 500 µm long |
| mm to µm | Multiply by 1000 | 0.04 mm × 1000 = 40 µm |
| µm to mm | Divide by 1000 | 250 µm ÷ 1000 = 0.25 mm |
Part 3 of 4Converting units step by step
1
Write down the image size and the actual size, including the unit beside each one.
2
Decide which unit you will work in. Millimetres is usually easiest because you measured the image with a ruler in mm.
3
If the actual size is in µm, divide it by 1000 to turn it into mm. If you prefer µm, multiply the mm value by 1000 instead.
4
Substitute the matching values into magnification = image size ÷ actual size.
5
Calculate the result. Magnification has no unit, so write it as, for example, ×25.
Part 4 of 4Worked example
1The question
- The question A student measures the image of a leaf section as 50 mm.
- The real leaf is 2000 µm thick.
- Calculate the magnification.
2Convert the units
- Convert the units Convert the actual thickness to mm: 2000 µm ÷ 1000 = 2 mm.
3Apply the equation
- Apply the equation magnification = image size ÷ actual size = 50 mm ÷ 2 mm = 25.
4State the answer
- State the answer The magnification is ×25, with no unit.
- Alternatively you could convert 50 mm into 50 000 µm and divide by 2000 µm, which also gives 25.
Key terms
| Term | Meaning |
|---|---|
| Millimetre (mm) | One thousandth of a metre; the unit normally used when measuring an image with a ruler. |
| Micrometre (µm) | One thousandth of a millimetre; the unit used to give the real size of cells and organelles. |
| Unit conversion | Changing a measurement from one unit to another without changing its actual size. |
| Magnification | The ratio of image size to actual size, written without units. |
Avoid these mistakes
1
Multiplying when you should dividegoing from µm to mm makes the number smaller, so divide by 1000.
2
Using 100 instead of 1000 — that is the factor for centimetres and millimetres, not millimetres and micrometres.
3
Putting mm and µm values into the same equation without converting, which gives an answer that is 1000 times too large or too small.
4
Writing a unit after the final magnification; it should be ×25, not 25 mm.
5
Not checking the answer is sensiblea microscope drawing of a cell should normally have a magnification in the tens, hundreds or thousands.
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