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This lesson covers the principles and techniques of microscopy as required by the Edexcel GCSE Biology specification (1BI0), Topic 1: Key Concepts in Biology. You need to understand the differences between light and electron microscopes, be able to use the magnification formula, perform unit conversions, and describe Core Practical 1 — using a light microscope to observe cells.
Most cells are too small to be seen with the naked eye. Microscopes are instruments that produce a magnified image of a small object, allowing us to observe structures that would otherwise be invisible.
Two key terms you must understand:
Exam Tip: Magnification and resolution are different things. A microscope can have high magnification but poor resolution, meaning the image is large but blurry. Resolution determines the level of detail you can see.
A light microscope (also called an optical microscope) uses visible light and glass lenses to magnify specimens.
| Property | Value |
|---|---|
| Maximum useful magnification | Up to ×1500 |
| Resolution | ~200 nm (0.2 μm) |
| Type of image | Can view living or dead specimens |
| Colour | Specimens can be viewed in colour (stains can be used) |
| Cost | Relatively inexpensive |
| Size | Small, portable |
Example: If the eyepiece lens is ×10 and the objective lens is ×40:
Total magnification = 10 × 40 = ×400
| Part | Function |
|---|---|
| Eyepiece lens | Magnifies the image (usually ×10) |
| Objective lenses | Provide different magnifications (typically ×4, ×10, ×40) |
| Stage | Platform where the slide is placed |
| Focusing knobs | Coarse focus for initial focusing; fine focus for sharp adjustment |
| Light source / mirror | Illuminates the specimen from below |
| Diaphragm / iris | Controls the amount of light passing through the specimen |
Electron microscopes use a beam of electrons instead of light to form an image. Because the wavelength of electrons is much shorter than visible light, electron microscopes have much higher resolution and can reveal much finer detail.
There are two main types:
| Property | Value |
|---|---|
| Magnification | Up to ×500,000 or more |
| Resolution | ~0.2 nm (1000× better than light microscopes) |
| Image type | 2D, thin cross-section images |
| Specimen | Must be very thin sections (electrons pass through the specimen) |
| Colour | Black and white (can be artificially coloured) |
| Property | Value |
|---|---|
| Magnification | Up to ×100,000 |
| Resolution | ~2–5 nm |
| Image type | 3D surface images |
| Specimen | Electrons are bounced off the surface |
| Colour | Black and white (can be artificially coloured) |
| Feature | Light Microscope | TEM | SEM |
|---|---|---|---|
| Maximum magnification | ×1,500 | ×500,000+ | ×100,000 |
| Resolution | ~200 nm | ~0.2 nm | ~2–5 nm |
| Image type | 2D (colour possible) | 2D (black and white) | 3D surface (black and white) |
| Living specimens? | Yes | No | No |
| Cost | Low | Very high | Very high |
| Size | Small, portable | Large, laboratory-based | Large, laboratory-based |
| Sample preparation | Minimal | Extensive (thin sections, vacuum) | Extensive (coated, vacuum) |
Exam Tip: Electron microscopes allowed scientists to see organelles like ribosomes, the internal structure of mitochondria (cristae), and the endoplasmic reticulum for the first time. Before electron microscopes, these structures were unknown.
The magnification formula is essential for calculations:
Magnification = Image size ÷ Actual size
This can be rearranged into a triangle (the MAI triangle):
I
-----
M × A
Exam Tip: To use the triangle, cover the quantity you want to find. If the remaining two are side by side, multiply them. If one is above the other, divide (top ÷ bottom).
A cell has an actual length of 50 μm. Under a microscope, the image of the cell measures 25 mm. Calculate the magnification.
Step 1: Convert both measurements to the same units.
25 mm = 25 × 1000 = 25,000 μm
Step 2: Apply the formula.
Magnification = Image size ÷ Actual size = 25,000 μm ÷ 50 μm = ×500
An image of a bacterium is 30 mm long at a magnification of ×10,000. Calculate the actual size of the bacterium.
Step 1: Apply the formula.
Actual size = Image size ÷ Magnification = 30 mm ÷ 10,000 = 0.003 mm
Step 2: Convert to appropriate units.
0.003 mm × 1000 = 3 μm
A mitochondrion is 2 μm long. It is viewed at ×5000 magnification. What size will the image be?
Image size = Magnification × Actual size = 5000 × 2 μm = 10,000 μm
Convert to mm: 10,000 ÷ 1000 = 10 mm
Exam Tip: Always convert to the same units before using the magnification formula. The most common error is mixing mm and μm. Convert image sizes (usually in mm) to μm by multiplying by 1000.
Very small measurements are often expressed in standard form (scientific notation):
Standard form is written as: A × 10ⁿ where 1 ≤ A < 10 and n is an integer.
Worked Example:
Express 0.0045 mm in standard form:
0.0045 = 4.5 × 10⁻³ mm
Convert this to μm: 0.0045 mm × 1000 = 4.5 μm = 4.5 × 10⁰ μm (or simply 4.5 μm)
Convert to nm: 4.5 μm × 1000 = 4500 nm = 4.5 × 10³ nm
This is one of the Edexcel Core Practicals that you must know in detail.
To use a light microscope to observe plant and animal cells.
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