Microscopes and magnification Revision Notes for AQA GCSE Biology Combined Science

Microscopes and magnification

Types of microscopes

There are two main types of microscopes that scientists use to study cells and tiny structures.

Light microscopes have been helping scientists for around 350 years. They use light rays to create enlarged images of small objects like cells. The best light microscopes can make things appear up to 2000 times bigger than they really are (written as ×2000).

Electron microscopes are much newer - they were developed in the last century. Instead of light, they use electrons to create images. These powerful microscopes can magnify objects up to 2 million times their actual size.

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The main advantage of electron microscopes is that they help scientists see the tiny structures inside cells that are impossible to see with light microscopes. This breakthrough has revolutionised our understanding of cell biology and molecular structures.

Resolving power

Resolving power tells us how well a microscope can separate two objects that are very close together. Think of it as the microscope's ability to show clear, sharp details.

Resolving power is measured in nanometres (nm). The smaller the number, the better the resolving power.

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

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This means electron microscopes can show much finer details than light microscopes. The better resolving power of electron microscopes allows scientists to see structures that are completely invisible under light microscopes.

Calculating magnification

Magnification shows how many times bigger an image appears compared to the real object.

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The key formula to remember is:

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

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

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

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Worked Example: Calculating Magnification

A light microscope has a ×10 eyepiece lens and a ×40 objective lens. An onion cell image measures 0.2 mm long.

(a) Calculate the total magnification: total magnification = 10 × 40 = ×400

(b) Find the real length of the cell: size of real object = 0.2 mm ÷ 400 = 0.0005 mm

Remember: 1 mm = 1000 μm, so 0.0005 mm = 0.5 μm

Scale bars

Many microscope images and diagrams include a scale bar. This is a line that shows you the actual size of structures in the image.

Scale bars help you:

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

Using scale bars to calculate magnification

To find magnification using a scale bar:

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

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Worked Example: Using Scale Bars

A cell diagram has a scale bar that measures 20 mm on the page. The scale bar represents 1 μm in real life.

Step 1: Length of scale bar = 20 mm = 20 × 1000 = 20,000 μm Step 2: Real length = 1 μm Step 3: Magnification = 20,000 ÷ 1 = ×20,000

Important unit conversions

When working with microscopes, you'll often need to convert between units:

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Essential Unit Conversion:

1 millimetre (mm) = 1000 micrometres (μm)

Always use the same units when doing calculations. It's usually easier to work in millimetres (mm) rather than centimetres (cm).

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Key Points to Remember:

Light microscopes can magnify up to ×2000, electron microscopes can magnify up to ×2 million
Magnification formula: $\frac{\text{size of image}}{\text{size of real object}}$
Electron microscopes have much better resolving power than light microscopes
Scale bars help you calculate magnification and find real sizes of objects
Always convert units properly: 1 mm = 1000 μm

Microscopes and magnification (AQA GCSE Biology Combined Science): Revision Notes