In simple terms
A friendly intro before the formal notes — no formulas yet.
Electromagnetic spectrum
Cambridge 9702 Paper 2 - Electromagnetic spectrum (7.4). Senpai Corner diagram-backed pilot with premium structure and live visuals.
- 1
7.4 Electromagnetic spectrum.
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Electromagnetic waves are transverse waves.
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It consists of electric field and magnetic field components.
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It can propagate without the need of a medium to carry them unlike mechanical waves.
What this topic covers
The official Cambridge syllabus points this lesson works through.
- 7.4.1
State that all electromagnetic waves are transverse waves that travel with the same speed c in free space
- 7.4.2
Recall the approximate range of wavelengths in free space of the principal regions of the electromagnetic spectrum from radio waves to -rays
- 7.4.3
Recall that wavelengths in the range 400–700 nm in free space are visible to the human eye
Explore the concept
Use the live diagram, PhET or GeoGebra sim, and synced steps — play it, drag controls, or tap a step.
Step-synced diagram — highlights what to look for in the simulation above.
Step 1
7.4 Electromagnetic spectrum.
11 more simulations for this topic — run them in the Simulations section below
Simulations
Every simulation here runs the real model — try the steps on a card, then check what you see against the notes.
11 simulations · 4 to start with
Start herein this order — each one shows a different piece of the topic
- oPhysicsStart here · 19702 7.4 · IB C.2
Electromagnetic Waves
Animated EM wave with oscillating E (green) and B (red) field vectors perpendicular to propagation
Why this one: E and B oscillate in phase, at right angles to each other and to the direction of travel: transverse.
Try this
- Watch the E and B vectors at one point over a cycle.
- Compare the directions of E, B and propagation.
Look for E and B oscillate in phase, perpendicular to each other and to the direction of travel.
Simulation by Tom Walsh, oPhysics.com — made with GeoGebra · Licensed to MarkScheme (site: free for non-profit educational use; applets made with GeoGebra)
- PhETStart here · 29702 7.4
Blackbody Spectrum
Slide the temperature of a glowing body and watch its spectrum move across the EM bands.
Why this one: Slide the temperature and the peak wavelength moves across the bands; read λ_peak and compare with the visible range.
Try this
- Set the Sun (5800 K) — note where the peak sits relative to the visible band.
- Drop to 3000 K (a light bulb) — the peak slides into the infrared.
- Turn on “Labels” and read the peak wavelength; multiply by T.
Look for Hotter → shorter peak wavelength (Wien); the visible band is a sliver of the spectrum.
Simulation by PhET Interactive Simulations, University of Colorado Boulder · Licensed to MarkScheme (public licence CC BY-NC 4.0 since 2026-03-30)
- PhETStart here · 39702 7.4
Color Vision
Shine red, green and blue bulbs, or a single filtered beam, at a viewer and see the colour they perceive.
Why this one: Drag the filter across 400–700 nm: visible light is one narrow band of the same spectrum.
Try this
- Turn all three RGB bulbs to full — the viewer sees white.
- Switch to the single bulb and drag the filter across the spectrum — one wavelength band passes.
- Narrow the filter — the colour gets purer but dimmer.
Look for Visible light is the 400–700 nm band of the EM spectrum; colours mix additively.
Simulation by PhET Interactive Simulations, University of Colorado Boulder · Licensed to MarkScheme (public licence CC BY-NC 4.0 since 2026-03-30)
- PhETStart here · 4Java · best on a laptop9702 7.4 · IB C.2
Radio Waves & Electromagnetic Fields
Oscillate the electron in a transmitting antenna, by hand or sinusoidally, and watch the radiated electric field reach the receiver.
Why this one: An oscillating charge radiates; raise its frequency and the wavelength shortens since λ = c/f.
Try this
- Drag the transmitter electron up and down by hand — watch the field ripple outward.
- Switch to sinusoidal oscillation and raise the frequency — the wavelength shortens.
- Watch the receiver electron — it copies the transmitter with a delay.
Look for An accelerating charge radiates; the wave travels at c, so λ = c/f.
Simulation by PhET Interactive Simulations, University of Colorado Boulder · Licensed to MarkScheme (public licence CC BY-NC 4.0 since 2026-03-30)
More simulations7 more on this topic — core ones first
- PhETCoreIB B.2 · 9702 7.4
Molecules and Light
Shine microwave, infrared, visible or UV light at CO₂, H₂O, N₂, O₂ and other molecules and see which absorb and vibrate.
Try this
- Pick CO₂ and infrared — the molecule absorbs and bends or stretches.
- Keep infrared and switch to N₂ or O₂ — the photons pass straight through.
- Switch to visible light with CO₂ — no absorption.
Look for Greenhouse gases absorb infrared but not visible light; N₂ and O₂ absorb neither — that selectivity is the greenhouse effect.
Simulation by PhET Interactive Simulations, University of Colorado Boulder · Licensed to MarkScheme (public licence CC BY-NC 4.0 since 2026-03-30)
- SimuPhysicsCore9702 7.4 · 9702 7.5 · IB C.2
Electromagnetic Waves
The electric and magnetic field vectors of a wave are drawn in 3D as it propagates
Try this
- Rotate the view and compare the two field directions.
- Compare both with the direction of travel.
- Follow one field through a cycle.
Look for The electric field, the magnetic field and the direction of travel are mutually perpendicular.
Open on SimuPhysicsRuns on their siteSimuPhysics by Mohamed Abdelsalam · Licensed to MarkScheme (site publishes no licence; served with frame-ancestors self)
- 3JCN PhysicsCore9702 7.4 · 9702 7.1 · 9702 7.2
Electromagnetic Waves
Watch oscillating E and B fields propagate as an electromagnetic wave
Try this
- Watch the E and B fields propagate.
- Compare the directions of E, B and the direction of travel.
- Note where E and B peak together.
Look for E and B oscillate in phase, perpendicular to each other and to the direction of propagation.
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
- PhETJava · best on a laptop9702 7.4 · IB C.2
Microwaves
Send microwaves at water molecules and watch them rotate; view the oscillating electric field as arrows or a curve.
Try this
- Fire a single microwave at one molecule — it twists with the field.
- Raise the frequency — the molecule flips faster.
- Fill the box with molecules — they all align and rotate together.
Look for An EM wave is an oscillating field; polar molecules absorb energy by rotating with it — the microwave-oven principle.
Simulation by PhET Interactive Simulations, University of Colorado Boulder · Licensed to MarkScheme (public licence CC BY-NC 4.0 since 2026-03-30)
- 3JCN Physics9702 7.4 · IB C.2
Heinrich Hertz's Experiment
Spark a transmitter and detect the EM wave at a receiving loop, as Hertz did
Try this
- Spark the transmitter and watch the receiving loop.
- Move the receiving loop farther away and compare the response.
Look for A spark in the transmitter induces a spark in the distant loop, showing that electromagnetic waves travel through space.
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
- 3JCN Physics9702 7.4 · IB C.2
Huygen measured the speed of light
Reproduce Romer/Huygens' speed-of-light estimate from Jupiter's moons
Try this
- Time the eclipses of Jupiter's moon when Earth is near Jupiter.
- Time them again when Earth is far away and compare the delay.
Look for The eclipse delay equals the extra distance divided by the speed of light.
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
Key formulas
Tap any symbol to reveal exactly what it means and its units.
Tap a symbol — great for exam definitions
Full topic notes
Formal explanation with the rigour you need for the exam.
The Universal Properties of EM Waves
Every electromagnetic wave shares a few core characteristics. Firstly, they are all transverse waves, meaning their oscillations are perpendicular to the direction they travel. Unlike sound waves, EM waves don't need a medium; they can travel through the vacuum of space. Most importantly, all EM waves travel at the same incredible speed in a vacuum.
Where is the speed of light in a vacuum (), is the frequency of the wave, and λ (lambda) is its wavelength. This fundamental relationship means that as the wavelength gets shorter, the frequency must increase to maintain a constant speed.
Exploring the Electromagnetic Spectrum
The electromagnetic spectrum is an ordered arrangement of these waves by their wavelength and frequency. Moving from the longest wavelengths (and lowest frequencies) to the shortest (and highest frequencies), the order is: Radiowaves, Microwaves, Infrared, Visible Light, Ultraviolet, X-Rays, and Gamma Rays. Each region has unique uses and properties due to its energy level, which is directly proportional to its frequency.
Components of the EM Spectrum in Detail
Each part of the electromagnetic spectrum is defined by a specific range of wavelengths and frequencies, which in turn dictates its properties and applications. Understanding these distinctions is crucial for applying physics concepts to real-world technology and interpreting astronomical observations.
Here is a breakdown of the principal components, their approximate wavelengths, and common uses:
- Radiowaves: Wavelengths > 10 cm. They have the lowest energy and are used for broadcasting radio and television signals, as well as in radio astronomy.
- Microwaves: Wavelengths from 1 mm to 30 cm. These are used in microwave ovens to heat food, for satellite communication, mobile phone networks, and radar systems.
- Infrared (IR): Wavelengths from 700 nm to 1 mm. Emitted by all objects with thermal energy, it is used in thermal imaging cameras, remote controls, and for data transmission in optical fibres.
- Visible Light: A narrow band with wavelengths from approximately 400 nm (violet) to 700 nm (red). This is the only portion of the spectrum that the human eye can detect.
- Ultraviolet (UV): Wavelengths from 10 nm to 400 nm. It has enough energy to cause chemical reactions, such as tanning and sunburn, and is used for sterilising equipment and in security marking.
- X-rays: Wavelengths from 0.01 nm to 10 nm. Their high energy and penetrating ability make them ideal for medical imaging (radiographs) and airport security scanners.
- Gamma Rays: Wavelengths < 0.01 nm. These are the most energetic and penetrating waves, produced by nuclear reactions. They are used in radiotherapy to destroy cancer cells and to sterilise medical instruments.
7.4 Electromagnetic spectrum.
Electromagnetic waves are transverse waves.
It consists of electric field and magnetic field components.
It can propagate without the need of a medium to carry them unlike mechanical waves.
The speed that electromagnetic waves travel at is 3.00 x 10⁸ m s^{-1}.
If this number seems familiar it’s because that’s the speed of light. Light is a wave or more specifically an electromagnetic wave.
A Special Property: Polarisation
One key feature that distinguishes EM waves as transverse waves is their ability to be polarised. Polarisation involves restricting the oscillations of the wave to a single plane. Imagine shaking a rope through a picket fence; only the shakes parallel to the fence gaps can pass through. This phenomenon is direct evidence that EM waves are not longitudinal, but rather vibrate perpendicular to their direction of travel.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
A certain radio wave has a frequency of 1.5 x 10⁷ Hz. Calculate its wavelength in a vacuum.
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Recall the wave equation for EM waves in a vacuum:
Green light has a typical wavelength of 550 nm. Calculate its frequency.
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Start with the wave equation for electromagnetic waves: .
How it all connects
The big idea sits in the middle — tap a linked idea to explore the link.
Tap a linked idea to see how it connects back to the main topic — that connection is what examiners reward.
Glossary
Key terms for this topic — skim now; the Check step will test them.
- Radiowaves
Wavelengths > 10 cm. They have the lowest energy and are used for broadcasting radio and television signals, as well as in radio astronomy.
- Microwaves
Wavelengths from 1 mm to 30 cm. These are used in microwave ovens to heat food, for satellite communication, mobile phone networks, and radar systems.
- Infrared (IR)
Wavelengths from 700 nm to 1 mm. Emitted by all objects with thermal energy, it is used in thermal imaging cameras, remote controls, and for data transmission in optical fibres.
- Visible Light
A narrow band with wavelengths from approximately 400 nm (violet) to 700 nm (red).
- Ultraviolet (UV)
Wavelengths from 10 nm to 400 nm. It has enough energy to cause chemical reactions, such as tanning and sunburn, and is used for sterilising equipment and in security marking.
- X-rays
Wavelengths from 0.01 nm to 10 nm. Their high energy and penetrating ability make them ideal for medical imaging (radiographs) and airport security scanners.
- Gamma Rays
Wavelengths < 0.01 nm. These are the most energetic and penetrating waves, produced by nuclear reactions.
Quick check
Write your answer first, then compare it with the model one — the gap is what you would have lost.
Teach it back
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Teach it back
Explain this topic as if teaching a friend. We name the gaps an examiner would still dock.
Revision flashcards
Guess first, then flip — retrieval beats re-reading.
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
7.4 Electromagnetic spectrum.
Electromagnetic waves are transverse waves.
It consists of electric field and magnetic field components.
It can propagate without the need of a medium to carry them unlike mechanical waves.
The speed that electromagnetic waves travel at is 3.00 x 10⁸ m s^{-1}.
If this number seems familiar it’s because that’s the speed of light. Light is a wave or more specifically an electromagnetic wave.
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
Mark a past-paper question on this topic
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Extra simulations & links
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Checkpoint
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