Microscopes and magnification Revision Notes for AQA GCSE Biology

Microscopes and magnification

Types of microscopes

There are two main types of microscopes you need to know about.

Light microscopes were invented around 350 years ago. They use light to create magnified images of cells and other tiny objects. The best light microscopes can magnify objects up to 2000 times (written as x2000).

Electron microscopes were invented much more recently, in the last century. These use electrons instead of light to create images. They are much more powerful and can magnify objects up to 2 million times.

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The development of electron microscopes revolutionised cell biology by allowing scientists to observe cellular structures that were previously invisible. This technological advancement opened up entirely new areas of biological research.

The higher magnification and better resolving power of electron microscopes lets scientists see and study structures inside cells that are too small to see with light microscopes.

Resolving power

Resolving power is how well a microscope can distinguish between two objects that are very close together. The better the resolving power, the clearer and more detailed the image will be.

Resolution is measured in nanometers (nm). The smaller the number, the better the resolution:

Light microscope: 200 nm resolution
Electron microscope (scanning): 10 nm resolution
Electron microscope (transmission): 0.1 nm resolution

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Resolution determines the level of detail you can see in microscope images. Electron microscopes can reveal cellular structures that are completely invisible under light microscopes due to their superior resolving power.

This means electron microscopes can see much finer details than light microscopes.

Magnification calculations

You can work out magnification using this formula:

$\text{magnification} = \frac{\text{size of image}}{\text{size of real object}}$

You can rearrange this formula to find the real size of an object:

$\text{size of real object} = \frac{\text{size of image}}{\text{magnification}}$

Important unit conversions:

1 mm = 1000 μm (micrometres)
Always convert to the same units before calculating

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Worked Example: Finding Real Cell Size

A cell image is 0.2 mm long under x400 magnification.

Step 1: Convert image size to micrometres 0.2 mm = 0.2 × 1000 = 200 μm

Step 2: Apply the formula Real cell size = 200 ÷ 400 = 0.5 μm

Scale bars

Diagrams and photos from microscopes often include a scale bar. This shows you the real size that distances in the image represent.

Scale bars help you:

See the real sizes of different objects in the image
Calculate the magnification if you don't know it

To calculate magnification using a scale bar:

Measure the length of the scale bar on the diagram (in mm)
Convert this to micrometres (multiply by 1000)
Use the formula: $\text{magnification} = \frac{\text{measured length}}{\text{real length}}$

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Worked Example: Calculating Magnification from Scale Bar

Scale bar shows 1 μm but measures 20 mm on the diagram.

Step 1: Convert measured length to micrometres 20 mm = 20 × 1000 = 20,000 μm

Step 2: Apply the magnification formula Magnification = 20,000 ÷ 1 = x20,000

Practice examples

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Worked Example 1: Total Magnification Calculation

A light microscope has x10 eyepiece and x40 objective lenses. A cell image is 0.2 mm long.

Step 1: Calculate total magnification Total magnification = 10 × 40 = x400

Step 2: Find real cell size Real cell size = 0.2 mm ÷ 400 = 0.0005 mm = 0.5 μm

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Worked Example 2: Scale Bar Analysis

A cell diagram has a scale bar showing 1 μm that measures 20 mm.

Step 1: Convert scale bar measurement Scale bar measurement = 20 × 1000 = 20,000 μm

Step 2: Calculate magnification Magnification = 20,000 ÷ 1 = x20,000

Summary

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Key Points to Remember:

Light microscopes magnify up to x2000, electron microscopes up to x2 million
Resolution: Light = 200nm, Electron scanning = 10nm, Electron transmission = 0.1nm
Formula: $\text{magnification} = \frac{\text{image size}}{\text{real object size}}$
Units: Always convert to same units (1mm = 1000μm)
Scale bars help you work out real sizes and magnifications

Microscopes and magnification (AQA GCSE Biology): Revision Notes