Microscope Skills (Copy)

Microscope Skills – Expanded Section (O Level & IGCSE Biology Alternate to Practical)

1. Parts of a Light Microscope (you must recognize and label)

• Eyepiece lens (usually ×10)
• Objective lenses (commonly ×4, ×10, ×40)
• Stage – where the slide is placed
• Clips – hold the slide in place
• Mirror or light source – provides illumination
• Coarse focus knob – large adjustments for focusing
• Fine focus knob – precise adjustments
• Diaphragm – controls light intensity

2. Using the Microscope – Step-by-Step

1. Place the slide securely on the stage using clips.
2. Start with the lowest power objective lens (×4 or ×10).
3. Adjust the mirror or diaphragm for appropriate light.
4. Use the coarse focus to bring the image roughly into view.
5. Switch to higher magnification if needed.
6. Use the fine focus to sharpen the image.
7. Draw and label what you observe.

3. Calculating Total Magnification

Formula:
Total magnification = eyepiece lens × objective lens

Examples:

• Eyepiece ×10 and objective ×4 → 10 × 4 = ×40
• Eyepiece ×10 and objective ×40 → 10 × 40 = ×400

4. Measuring Cell Size

To estimate the size of cells, use the field of view (FOV) and number of cells visible:

Step-by-Step:

1. Measure the diameter of the field of view (given or estimated using a graticule).
2. Count how many cells fit across the FOV.
3. Estimate size of one cell = FOV diameter / number of cells across

Example:
If FOV = 2 mm and 4 cells fit across →
Size of one cell = 2 mm / 4 = 0.5 mm = 500 µm

5. Units Conversion

• 1 mm = 1000 µm (micrometres)
• 1 µm = 1000 nm (nanometres)

Example conversions:

• 2 mm = 2000 µm
• 0.03 mm = 30 µm

6. Drawing What You See

• Use clear, unshaded pencil lines
• Only draw what is visible under the given magnification
• Label key features:
  • For onion cell: cell wall, nucleus, cytoplasm, vacuole
  • For leaf section: epidermis, palisade, spongy layer, stomata
• Use a title like “View of onion cell under ×100 magnification”
• State magnification below drawing

7. Describing Microscope Observations

Typical phrasing:

• “Cells are rectangular with thick walls (cell wall visible).”
• “Nucleus is visible as a dense, dark-stained circle.”
• “Chloroplasts appear as small green discs.”
• “Air spaces seen between loosely arranged spongy mesophyll cells.”

8. Common Practical Tasks

9. Calculating Actual Size

Formula:
Actual size = Image size / Magnification

Important: Convert image size to micrometres (µm) before using the formula.

Example:
If image size = 60 mm = 60,000 µm and magnification = ×400
→ Actual size = 60,000 µm / 400 = 150 µm

Common Mistakes to Avoid

• Starting with high power → always start with low
• Using ink → must draw in pencil
• Incorrect magnification formula
• Mixing up image size and actual size
• Drawing things not visible under microscope
• Forgetting labels, title, or magnification statement

Marking Points for Microscope Skills