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AQA GCSE Physics · Topic 4.6 Waves · Electromagnetic waves
The Electromagnetic Spectrum: Order, Uses and Hazards
In this lesson you'll learn to: put the seven groups of the electromagnetic spectrum in order of
wavelength, frequency and energy, state what all electromagnetic waves have in common, give a
use and a hazard for each group, explain why ultraviolet, X-rays and gamma rays are the
dangerous ones, and calculate a wavelength from a frequency in GHz.
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Prefer to watch? Scroll to the bottom for the video on this topic
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A video walkthrough is coming soon — it will appear at the bottom of this page
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Start here — play the electromagnetic spectrum game
Don't read anything yet. Pick where to start (longest wavelength, highest frequency, lowest energy…) and tap the seven groups in order. Get one wrong and the buzzer goes. Then play it with the uses, then with the hazards, and beat your best time. Turn the sound off with the button at the top if you're in a library.
The Electromagnetic Spectrum Game
Start with the
Tap the four statements that are true for every electromagnetic wave.
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▲ Four tabs: the order, the uses, the hazards, then what every EM wave shares
Beat the buzzer? Now learn it for the exam.
The notes below have the order both ways, the four things every EM wave shares, a use and a hazard for each group, and the GHz calculation that catches people out.
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Revision notes
The seven groups, in order
Electromagnetic waves form a continuous spectrum. We split it into seven groups. From the longest wavelength to the shortest:
⭐ One order, six ways to ask for it
Longest wavelength, lowest frequency and lowest energy all start at radio waves.
Shortest wavelength, highest frequency and highest energy all start at gamma rays.
Visible light is the only part our eyes can detect, and it's a very small part of the spectrum.
What all electromagnetic waves have in common
〰
Transverse
The oscillations are perpendicular to the direction of energy transfer.
⚡
Same speed in a vacuum
3 × 10⁸ m/s, whatever the group. They can all travel through a vacuum (space).
↩
Reflect and refract
They can all be reflected, and refracted when they change speed in a new material.
☀
Transfer energy
From the source of the waves to an absorber. Gamma carries the most energy, radio the least.
Uses and hazards
| Group | Uses | Hazard |
|---|---|---|
| Radio waves | broadcasting radio and TV; communications | no known risks at normal levels |
| Microwaves | mobile phones; satellite communication; cooking food | internal heating of body tissue |
| Infrared | thermal imaging and night-vision cameras; heaters and grills; remote controls | skin burns |
| Visible light | seeing; photography; fibre-optic communication | very bright light or lasers permanently damage the retina, causing blindness |
| Ultraviolet | fluorescent lamps; security marking; sterilising water | damages skin cells: skin cancer and eye damage |
| X-rays | seeing broken bones; airport security scanners | ionising: mutates cells, can cause cancer |
| Gamma rays | sterilising surgical instruments and food; treating cancer | ionising: mutates cells, can cause cancer |
⭐ Ionising radiation: ultraviolet, X-rays and gamma rays
These three have enough energy to ionise atoms. That can mutate cells (damage the DNA), which can cause cancer.
Ultraviolet only reaches the skin: it doesn't travel deep into the body, so it causes skin cancer. X-rays and gamma rays pass deep into the body.
The higher the frequency, the more energy the wave carries and the more harm it can do. How much harm a dose could do is measured as a radiation dose in sieverts (Sv).
Be specific. "Used in hospitals" is not a use: say treating cancer or
seeing broken bones. "It's dangerous" is not a hazard: say internal heating of body tissue or
skin cancer.
The wave equation with a prefix
EM waves follow v = f × λ, with v = 3.0 × 10⁸ m/s. Frequencies are often given with a prefix, so convert first:
| Prefix | Means | Example |
|---|---|---|
| k (kilo) | × 10³ | 200 kHz = 2.0 × 10⁵ Hz |
| M (mega) | × 10⁶ | 100 MHz = 1.0 × 10⁸ Hz |
| G (giga) | × 10⁹ | 12 GHz = 1.2 × 10¹⁰ Hz |
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How to write it in the exam
Every "use" or "hazard" answer, the same shape
Name the group →
give the actual job or the actual harm →
if it asks why, link it to energy, ionising or how deep it goes.
which wavebe specificthe physics reason
Workbook pages 12–13 · the order
Which way do the wavelength and frequency change towards gamma rays?
- Towards gamma rays the wavelength decreases and the frequency increases.
- Towards radio waves the wavelength increases and the frequency decreases.
What do all electromagnetic waves have in common? They all travel at the same speed in a vacuum (3 × 10⁸ m/s); they can all be reflected and refracted; they are all transverse and transfer energy.
Exam practice · page 14 · Q1(a)–(c) · 5 marks
(a) The table shows Radio, ___, Infrared, Visible, ___, X-rays, Gamma rays. Name the two missing groups.
- Microwaves (1) and ultraviolet (1), each in the right box.
(b) The arrow points from radio to gamma. It shows: A increasing wavelength, B increasing frequency, C increasing wave speed, D decreasing energy.
- B. A and D are backwards; C is wrong because every EM wave travels at the same speed in a vacuum.
(c) State two properties that all electromagnetic waves have in common.
- Any two: all transverse · all the same speed in a vacuum (3 × 10⁸ m/s) · all travel through a vacuum · all transfer energy.
Exam practice · page 14 · Q1(d) · 3 marks
A satellite TV signal is carried by microwaves with a frequency of 12 GHz. The speed of electromagnetic waves is 3.0 × 10⁸ m/s. Calculate the wavelength.
- 12 GHz = 12 × 10⁹ = 1.2 × 10¹⁰ Hz (1)
- λ = v ÷ f = 3.0 × 10⁸ ÷ 1.2 × 10¹⁰ (1)
- λ = 0.025 m (2.5 cm) (1)
Forget to convert the GHz and you lose a mark for a power-of-ten error.
Exam practice · page 14 · Q1(e) · 3 marks
(i) Give one use of gamma rays in a hospital.
- Sterilising medical equipment, or treating cancer (radiotherapy), or as a tracer to detect cancer.
(ii) A radiographer takes X-ray images every day. Explain why they stand behind a lead screen.
- X-rays are ionising: they can damage cells and cause mutations (1).
- The radiographer is exposed many times a day, so the risk of cancer builds up; the lead absorbs the X-rays (1).
The patient has one image, so their dose is small. The second mark needs the repeated exposure or the lead absorbing the X-rays.
Exam practice · page 14 · Q1(f) and (g) · 4 marks
(f) Microwaves and infrared can both cook food. Describe how the harm each does to the body is different.
- Microwaves heat the body from the inside: internal heating of body tissue (1).
- Infrared heats the surface: it causes skin burns (1).
(g) A star gives out a burst of gamma rays and a burst of radio waves at the same moment. Which arrives first?
- They arrive at the same time (1),
- because all electromagnetic waves travel at the same speed in a vacuum (1).
"Gamma first because it has more energy" gets no marks.
📘 Now do it in your workbook
Pages 12 and 13: draw each group's wave, fill in the order, the arrows and what they all share, and choose a use and a hazard for every group. Page 14: a full 15-mark exam question on the whole spectrum.
AQA GCSE Physics Workbook · WAVES · pages 12, 13 and 14The full workbook covers the whole of Topic 4.6 Waves, with exam-style questions and worked answers.
✅ Can you do it? Tick as you go
0% complete🎉 Nice work! You've ticked off every objective for this spec point.
Don't forget to hit “Mark complete” at the bottom of the lesson.
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Prefer to watch? Here's the whole thing
Video walkthrough — coming soon
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it.
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it. The video is on its way.
🎬
Video coming soon
The walkthrough for this lesson is being filmed and will appear right here. Until then, the animation and the notes above cover every mark.
Now put it into practice
Pages 12 to 14 of the workbook have the whole spectrum, a use and a hazard for every group, and a 15-mark exam question, with answers at the back.
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