Size of specimens
Section: Biology | Syllabus: Cambridge IGCSE Combined Science (0653)
The Magnification Formula
When viewing specimens under a microscope, magnification is used to describe how many times larger the image appears compared to the actual object.
- Magnification = (Image size/Actual size) - all measurements must be in the same units (usually millimetres, mm)
Rearranging the formula gives:
- Image size = Magnification × Actual size
- Actual size = (Image size/Magnification)
Magnification describes how many times larger the image is compared with the actual specimen
Worked Example: Finding Magnification
- Question: a cell appears 30 mm long in a diagram. Its actual length is 0.05 mm. What is the magnification?
- Answer: Magnification = (Image size/Actual size) = (30 mm/0.05 mm) = × 600
Worked Example: Finding Actual Size
- Question: a drawing of a cell is 45 mm in diameter and the magnification is ×300. What is the actual diameter of the cell?
- Answer: Actual size = (Image size/Magnification) = (45 mm/300) = 0.15 mm
Worked Example: Finding Image Size
- Question: a bacterium is 0.002 mm long. If it is drawn at ×2000, how long will the drawing be?
- Answer: Image size = Magnification × Actual size = 2000 × 0.002 mm = 4 mm
Converting Millimetres and Micrometres (Supplement)
Cells and organelles are often measured in micrometres (µm) because they are very small. Measurements must sometimes be converted between mm and µm before the magnification formula can be used.
- 1 mm = 1000 µm | 1 µm = 0.001 mm
| To convert | Operation | Example |
|---|---|---|
| mm → µm | Multiply by 1000 | 0.05 mm × 1000 = 50 µm |
| µm → mm | Divide by 1000 | 75 µm ÷ 1000 = 0.075 mm |
Worked Example: Magnification Calculation with Unit Conversion
- Question: a cell has an actual diameter of 80 µm. In a micrograph it measures 40 mm. Calculate the magnification.
- Step 1: convert the actual size to the same units as the image size: 80 µm ÷ 1000 = 0.08 mm
- Step 2: Magnification = (Image size/Actual size) = (40 mm/0.08 mm) = × 500
Common Mistakes
- ❌ Applying the magnification formula without checking units first
- ✓ image size and actual size must be in the same unit before dividing. Convert everything to mm (or everything to µm) first - a mismatch in units is the most common source of error in magnification questions
Interactive revision notes, videos and practice questions load below.