2.1.3Organisation of the Organism
Magnification Formula — Cambridge IGCSE Co-ordinated Sciences 0654 - Biology Revision Notes & Topical Questions
5 min read 6 parts Lesson 4 of 5
Section 1 of 6Why we calculate magnification
Seeing the very small
- Seeing the very small Most cells and organelles are far too small to see with the naked eye,
- so we view them through a microscope that makes the image larger than the real object.
- A drawing or photograph of a specimen is therefore bigger than the specimen itself.
Comparing image and object
- Comparing image and object Magnification tells us how many times larger the image is than the real specimen.
- If a cell is magnified 100 times, a cell that is 0.05 mm across will appear 5 mm across in the image.
Where the skill is tested
- Where the skill is tested Magnification questions appear in theory papers, multiple-choice questions and the practical or alternative-to-practical papers,
- where you often measure a drawing with a ruler.
Section 2 of 6The equation and its three forms
| To find | Equation | Unit of answer |
|---|---|---|
| Magnification | magnification = image size ÷ actual size | None (written as ×50, 200× and so on) |
| Actual size | actual size = image size ÷ magnification | Same unit as the image size you used |
| Image size | image size = magnification × actual size | Same unit as the actual size you used |
Section 3 of 6Rearranging with an equation triangle
1
Draw a triangle and split it into three sections: put image size in the top section, with actual size and magnification side by side in the bottom section.
2
Cover the quantity you want to find with your finger.
3
If the two remaining quantities are side by side, multiply them. If one is above the other, divide the top one by the bottom one.
4
Check that the image size and actual size are measured in the same unit before you substitute any numbers.
5
Substitute, calculate, and give the answer with the correct unit (or with no unit if you found magnification).
Section 4 of 6Worked example
The question
- The question A drawing of an animal cell measures 30 mm with a ruler.
- The drawing has a magnification of ×3000.
- Calculate the actual size of the cell.
The working
- The working We want the actual size, so use: actual size = image size ÷ magnification.
- Actual size = 30 mm ÷ 3000 = 0.01 mm.
- Because 1 mm = 1000 µm (Extended content), this is the same as 10 µm, which is a sensible size for an animal cell.
Sense check
- Sense check The image must always be larger than the object when magnification is greater than 1,
- so the answer for actual size should be a small number.
- If your actual size comes out bigger than the image, you have probably divided the wrong way round.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Magnification | How many times larger an image appears compared with the real object; it has no units. |
| Image size | The size of the object as it appears in a drawing, photograph or microscope view, measured with a ruler. |
| Actual size | The real size of the specimen in life. |
| Equation triangle | A memory aid that shows how to rearrange an equation with three quantities. |
| Millimetre (mm) | A unit of length equal to one thousandth of a metre; the unit usually used when measuring drawings with a ruler. |
Section 6 of 6Exam technique
1
Writing a unit after a magnification such as '40 mm' — magnification is a ratio, so it is written as ×40 with no unit.
2
Substituting an image size in millimetres and an actual size in micrometres without converting one of them first.
3
Dividing the wrong way round, for example calculating actual size ÷ image size, which gives a magnification below 1 for an enlarged image.
4
Measuring the drawing in centimetres when the question expects millimetres — read the question carefully and measure in the unit asked for.
5
Forgetting to measure the length of the whole specimen from one end to the other and measuring only part of it.
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