Size of SpecimensCambridge IGCSE Biology: Revision notes
Section 1
What is magnification and how do we calculate it?
Magnification describes how many times larger an image appears compared to the actual specimen. The formula used to calculate magnification is:
magnification = image size ÷ actual size
Both the image size and actual size must be measured in the same units (usually millimetres or micrometres). The magnification value tells you how many times the image has been enlarged. For example, a magnification of ×400 means the image is 400 times larger than the actual specimen.
This formula can be rearranged to find missing values:
- image size = magnification × actual size
- actual size = image size ÷ magnification
Always state the magnification with the multiplication symbol, e.g. ×400, not just '400'. Examiners expect to see this notation in your answers.
Think of magnification like a photocopier zoom setting: ×2 makes something twice as big, ×10 makes it ten times bigger, but the actual object hasn't changed size.
Section 2
How do we measure specimens in millimetres?
When measuring biological specimens, millimetres (mm) is the standard unit used in most practical work. Measurements are typically taken using:
- Microscope graticules (eyepiece scales)
- Ruler measurements of photographs or printed images
- Digital measurement tools on microscope software
When you measure the image of a specimen under a microscope or in a photograph, you are measuring in millimetres. This is the image size. The actual size of the specimen may be much smaller and needs to be calculated using the magnification formula.
Important: Always record measurements to an appropriate number of decimal places, usually to 0.1 mm or 0.01 mm depending on the precision of your measuring instrument.
Students often forget to state the units (mm) when recording measurements. Always include the unit—this is essential for marking.
Section 3
How do we convert between millimetres and micrometres?
Micrometres (μm) are used to measure very small structures such as cells and organelles. You must be able to convert between millimetres and micrometres accurately.
Conversion factor:
- 1 mm = 1000 μm
- 1 μm = 0.001 mm (or 1/1000 mm)
To convert from mm to μm: multiply by 1000
- Example: 0.05 mm = 0.05 × 1000 = 50 μm
To convert from μm to mm: divide by 1000
- Example: 500 μm = 500 ÷ 1000 = 0.5 mm
| Conversion direction | Operation | Example |
|---|---|---|
| mm → μm | Multiply by 1000 | 0.025 mm = 25 μm |
| μm → mm | Divide by 1000 | 2500 μm = 2.5 mm |
When working with magnification calculations, ensure all measurements are in the same units before applying the formula. If one measurement is in mm and another in μm, convert one to match the other first.
Convert 0.3 mm to micrometres: 0.3 mm × 1000 = 300 μm. To check: 300 μm ÷ 1000 = 0.3 mm ✓
A useful memory aid: 'mm to μm, move the decimal three places right' (multiply by 1000). 'μm to mm, move the decimal three places left' (divide by 1000).
Section 4
How do we calculate the actual size of a specimen?
To find the actual size of a specimen, you need:
- The image size (measured directly from the microscope image or photograph in mm)
- The magnification at which the image was produced
Use the rearranged formula:
actual size = image size ÷ magnification
Note that magnification is written as ×400 (for example), but when using it in the formula, use only the number: 400.
Step-by-step process:
- Measure the image size in millimetres
- Note the magnification (e.g. ×400)
- Divide the image size by the magnification number
- The result is the actual size in millimetres
- Convert to micrometres if needed (multiply by 1000)
The actual size will always be much smaller than the image size because magnification enlarges the image significantly.
An onion cell appears 12 mm in the image at ×400 magnification. Actual size = 12 ÷ 400 = 0.03 mm = 30 μm. The real cell is much smaller than what you see on screen.
Common error: multiplying image size by magnification instead of dividing. Remember—the actual specimen is smaller than the image, so you must divide.
Section 5
How do we use the magnification formula in different scenarios?
The magnification formula can be applied in three different ways depending on what information you have and what you need to find:
| Scenario | Given information | Formula to use | Example |
|---|---|---|---|
| Find magnification | Image size and actual size | magnification = image size ÷ actual size | Image 8 mm, actual 0.02 mm → magnification = 8 ÷ 0.02 = ×400 |
| Find image size | Magnification and actual size | image size = magnification × actual size | ×600 magnification, actual 0.05 mm → image size = 600 × 0.05 = 30 mm |
| Find actual size | Image size and magnification | actual size = image size ÷ magnification | Image 15 mm, ×500 → actual size = 15 ÷ 500 = 0.03 mm |
Key points:
- Always ensure units match before calculating
- If units differ, convert one measurement first
- State your final answer with appropriate units (mm or μm)
- Show all working in your exam answer—marks are awarded for correct method as well as final answer
- Round final answers sensibly (usually to 2-3 significant figures)
In exam questions, identify which value you need to find, then select the correct version of the formula. Write the formula, show the numbers substituted in, then give the final answer with units.
Question: A mitochondrion is 2 μm long. Under ×2000 magnification, how long does it appear in the image? Convert μm to mm first: 2 μm = 0.002 mm. Image size = 2000 × 0.002 = 4 mm.
Must Know
- Magnification formula: magnification = image size ÷ actual size (rearrange as needed: image size = magnification × actual size; actual size = image size ÷ magnification)
- Unit consistency is critical: both image size and actual size must be in the same units (mm or μm) before using the formula
- Conversion between units: 1 mm = 1000 μm; multiply by 1000 to convert mm → μm; divide by 1000 to convert μm → mm
- Actual size is always smaller than the image size because magnification enlarges specimens; when calculating actual size, always divide by magnification
- Show all working in exam answers: write the formula, substitute the numbers, then state the final answer with units
- Common magnifications in practical work: ×100, ×400, ×600, ×1000; larger magnification = smaller actual size for the same image size
That's the notes covered.
Carry on to the next subtopic.