CIE iGCSE Biology-2.2 Size of specimens- Study Notes- New Syllabus
CIE iGCSE Biology-2.2 Size of specimens- Study Notes – New syllabus
CIE iGCSE Biology-2.2 Size of specimens- Study Notes -CIE iGCSE Biology – per latest Syllabus.
Key Concepts:
Core
- State and use the formula:
magnification = image size ÷ actual size - Calculate magnification and size of biological specimens using millimetres as units
Supplement
- Convert measurements between millimetres (mm) and micrometres (μm)
Size of Specimens and Magnification
Magnification is how much bigger an object appears compared to its actual size.
It tells you how many times the real specimen has been enlarged in a microscope image or diagram.
- If something is magnified 100×, it looks 100 times bigger than it really is.
- Magnification is just a number – it has no units but is often written with a multiplication sign (e.g., 100×).
📏 The Magnification Formula:
| Term | Definition |
|---|---|
| Magnification | The number of times the real object has been enlarged (no unit) |
| Image size | The size of the object in the picture or microscope image (in mm or µm) |
| Actual size | The true size of the object in real life (usually in µm or mm) |
Rearranged formulas:
\[ \text{Image size} = \text{Actual size} \times \text{Magnification} \]
🧪 Example 1: Calculate Magnification
A cell measures 45 mm in an image. The actual size of the cell is 15 µm.
Step 1: Convert units so they match
- Image size = 45 mm = 45,000 µm
- Actual size = 15 µm
📐 Unit Conversion Tip:
Always use the same units before applying the formula!
Common conversions:
• 1 mm = 1000 µm
• 1 cm = 10 mm
• 1 mm = 0.001 m
Step 2: Apply the formula
Answer: Magnification is 3000 times
🧪 Example 2: Find Actual Size
A photo shows a magnified image of a leaf cell. The image measures 60 mm and the magnification is 1500×. What is the actual size?
Step 1: Convert image size to µm → 60 mm = 60,000 µm
Step 2: Apply the formula
Answer: Actual size = 40 micrometres
🧪 Example 3: Find Image Size
A red blood cell has an actual size of 7 µm. It was magnified 2000×. What is its image size?
Image size = 7 × 2000 = 14,000 µm = 14 mm
Answer: Image size = 14 mm
🛑 Common Mistakes to Avoid:
| Mistake | Correction |
|---|---|
| Forgetting to convert units | Always convert both sizes to the same units (e.g., µm) |
| Writing units with magnification | Magnification has no unit – just write the number + × |
| Using ruler measurements without converting | Convert mm to µm if needed |
Applying Magnification Formula Using Millimetres (mm)
Many microscope diagrams include rulers in millimetres (mm) rather than micrometres (µm).
This method helps you calculate magnification more easily – without needing to convert units, unless specified.
📌 Step-by-Step Strategy (Using mm)
- Measure Image Size: Use a ruler to measure the specimen in the diagram.
Example: Image size = 12 mm (root hair cell) - Use Given Actual Size: Either provided or calculated from a scale bar.
Example: Actual size = 0.006 mm - Apply the Formula:
Magnification = Image size ÷ Actual size
Ensure both values are in millimetres.
🧪 Sample Problem 1:
Q: Image size = 20 mm
Actual size = 0.002 mm
Magnification = 20 ÷ 0.002 = 10,000×
🧪 Sample Problem 2:
Q: Image size = 16 mm
Magnification = ×400
Actual size = 16 ÷ 400 = 0.04 mm
🧪 Sample Problem 3:
Q: Actual size = 0.02 mm
Magnification = ×2500
Image size = 0.02 × 2500 = 50 mm
1 mm = 1000 µm
Stick to mm unless µm is needed
Use calculators for accurate decimals
🎯 Reminder:
| Do’s | Don’ts |
|---|---|
| Use the same unit (stick to mm) | Mix mm and µm without converting |
| Include working and formula | Skip calculation steps |
| Label answers clearly | Add units to magnification (no units) |
Converting Between Millimetres (mm) and Micrometres (μm)
🧠 Why Do We Convert?
- Microscopes often use micrometres (μm) because cells and tissues are very small.
- Diagrams and rulers often use millimetres (mm).
- To calculate magnification or real size, you must use the same units –so conversion is essential.
🔄 Conversion Rule:
🔁 How to Convert:
| To Convert… | You Must… | Example |
|---|---|---|
| mm → μm | Multiply by 1000 | 2 mm = 2 × 1000 = 2000 μm |
| μm → mm | Divide by 1000 | 5000 μm = 5000 ÷ 1000 = 5 mm |
🧪 Example 1: Convert mm to μm
Q: Convert 3.6 mm to micrometres.
A: 3.6 × 1000 = 3600 μm
🧪 Example 2: Convert μm to mm
Q: Convert 12,000 μm to mm.
A: 12,000 ÷ 1000 = 12 mm
🎯 Quick Conversion Table:
| mm | μm |
|---|---|
| 0.001 mm | 1 μm |
| 0.01 mm | 10 μm |
| 0.1 mm | 100 μm |
| 1 mm | 1000 μm |
| 10 mm | 10,000 μm |
🧠 Memory Tip:
“Milli means thousandth” and “micro is even smaller.”
So: 1 mm = 1000 μm
μm are tiny!
✅ Final Recap:
| Convert From | Do This | Example |
|---|---|---|
| mm to μm | Multiply by 1000 | 1.5 mm = 1500 μm |
| μm to mm | Divide by 1000 | 8000 μm = 8 mm |