Microscopy and Magnification

This lesson covers microscopy and magnification calculations as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand the differences between light and electron microscopes, use the magnification formula, and describe how to prepare slides and use a light microscope.

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Why Do We Need Microscopes?

Most cells are too small to see with the naked eye. Microscopes use lenses (or beams of electrons) to produce magnified images of small objects. In biology, microscopes allow us to see the internal structures of cells.

There are two key concepts:

• Magnification — how much larger the image is compared to the real object.
• Resolution — the ability to distinguish between two points that are very close together. Higher resolution means finer detail can be seen.

Exam Tip: Magnification and resolution are different things. A microscope can produce a very large image (high magnification) but if the resolution is low, the image will be blurry and you will not see fine detail.

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Light Microscopes

A light microscope uses visible light and glass lenses to magnify specimens.

Features

• Maximum useful magnification: approximately × 1500
• Maximum resolution: approximately 200 nm (0.2 μm)
• Can view whole cells and larger organelles (nucleus, chloroplasts, mitochondria — as dots)
• Specimens can be living or dead
• Relatively cheap and portable
• Specimens are often stained with dyes (e.g. iodine, methylene blue) to make structures visible

How to Use a Light Microscope (Core Practical)

1. Place the slide on the stage and secure it with the stage clips.
2. Select the lowest power objective lens.
3. Use the coarse focus wheel to bring the image roughly into focus.
4. Switch to a higher power objective lens for greater magnification.
5. Use the fine focus wheel to sharpen the image.
6. Adjust the light source or mirror for optimal illumination.

Preparing a Slide

1. Place a thin specimen on a clean glass slide.
2. Add a drop of water (to keep cells alive and prevent air bubbles).
3. Add a drop of stain if needed (e.g. iodine for plant cells, methylene blue for animal cells).
4. Carefully lower a coverslip at an angle to avoid trapping air bubbles.

Exam Tip: When describing slide preparation, always mention lowering the coverslip at an angle to avoid air bubbles. This is a common mark point in practical questions.

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Electron Microscopes

Electron microscopes use a beam of electrons instead of light. Because electrons have a much shorter wavelength than visible light, electron microscopes have far greater resolution.

Types of Electron Microscope

Feature	Transmission Electron Microscope (TEM)	Scanning Electron Microscope (SEM)
Beam passes	Through the specimen	Over the surface of the specimen
Image type	2D, high detail of internal structures	3D surface image
Maximum magnification	~× 2,000,000	~× 500,000
Maximum resolution	~0.1 nm	~1 nm

Comparison: Light vs Electron Microscopes

Feature	Light Microscope	Electron Microscope
Radiation source	Visible light	Beam of electrons
Maximum magnification	~× 1500	~× 2,000,000
Maximum resolution	~200 nm	~0.1 nm (TEM)
Specimen	Living or dead	Dead only (vacuum required)
Cost	Relatively cheap	Very expensive
Size	Small and portable	Large, requires a dedicated room
Colour	Can see natural colours or stains	Black and white (colour added digitally)

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The Magnification Formula

$\text{Magnification} = \frac{\text{Image size}}{\text{Actual size}}$Magnification=Actual sizeImage size​

This can be rearranged using the magnification triangle:

graph TD
    A["I (Image size)"] --- B["M (Magnification) × A (Actual size)"]

The three rearrangements are:

• Magnification = Image size ÷ Actual size
• Image size = Magnification × Actual size
• Actual size = Image size ÷ Magnification

Worked Example 1

A cell has an actual size of 50 μm. Under a microscope, the image measures 20 mm. Calculate the magnification.

Step 1: Convert to the same units.

20 mm = 20,000 μm

Step 2: Apply the formula.

Magnification = Image size ÷ Actual size = 20,000 ÷ 50 = × 400

Worked Example 2

A drawing of a cell is 60 mm wide. The magnification used was × 500. Calculate the actual size of the cell.

Actual size = Image size ÷ Magnification = 60 mm ÷ 500 = 0.12 mm = 120 μm

Exam Tip: Always convert both measurements to the same units before calculating. Show all working, including unit conversions, for full marks.

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Summary

• Light microscopes use visible light, have a maximum magnification of ~× 1500 and a resolution of ~200 nm. Specimens can be living or dead.
• Electron microscopes use a beam of electrons, have much higher magnification (~× 2,000,000) and resolution (~0.1 nm), but specimens must be dead.
• TEM produces 2D images of internal structures; SEM produces 3D surface images.
• The magnification formula: Magnification = Image size ÷ Actual size.
• Always convert to the same units before calculating.
• When preparing a slide, lower the coverslip at an angle to avoid trapping air bubbles.

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Deeper Look: Why Microscopes Changed Biology

Before the invention of the microscope in the 17th century, biologists did not know that cells existed. Antonie van Leeuwenhoek used a simple single-lens microscope to observe "animalcules" (bacteria and protozoa), and Robert Hooke coined the term "cell" when he looked at thin slices of cork. Since then, improvements in magnification and resolution have allowed scientists to see progressively smaller structures: cells → organelles → molecules. The development of the electron microscope in the 20th century was particularly revolutionary because it increased resolution a thousand-fold, revealing the detailed internal structure of mitochondria, chloroplasts and ribosomes for the first time.

Magnification vs Resolution — A Deeper Look

These two concepts are frequently confused, so it is worth spelling them out clearly:

• Magnification is the ratio of the image size to the actual size. It tells you how much larger the image appears compared to the real object.
• Resolution is the smallest distance between two points at which they can still be distinguished as separate. Higher resolution means more detail is visible.