Edexcel IGCSE Physics 3(c) The Electromagnetic Spectrum — Notes
Written for Pearson Edexcel International GCSE Physics (4PH1), Science Double Award (4SD0) and Science Single Award (4SS0), linear and modular.
Which parts do I need?
Everything on this page is for all three routes. The wording of 3(c) is identical in Single Award, Double Award and Physics, so no route skips anything here.
What this page covers
| Spec | Modular no. | You need to be able to… | Route |
|---|---|---|---|
| 3.10 | 5.10 | know that light belongs to a continuous family of electromagnetic waves, from radio to gamma rays, which all travel at the same speed in a vacuum | Single · Double · Physics |
| 3.11 | 5.11 | list the seven regions in order of wavelength and frequency, and the colours of visible light in order | Single · Double · Physics |
| 3.12 | 5.12 | explain how each region is used, linking the use to a property of the radiation | Single · Double · Physics |
| 3.13 | 5.13 | explain the harm that too much microwave, infrared, ultraviolet or gamma radiation does to the body, and describe simple ways to reduce the risk | Single · Double · Physics |
Modular candidates: Waves is topic 5 in the modular specifications, assessed in 4XPH1 Unit 2 (4WPH2) and 4XSD1 Unit 6 (4WSD6). The modular number is the same in both.
In one minute
- Electromagnetic waves form a continuous spectrum: radio, microwave, infrared, visible, ultraviolet, x-ray, gamma ray.
- In that order, wavelength decreases and frequency increases.
- Visible light runs red, orange, yellow, green, blue, indigo, violet — red has the longest wavelength.
- All electromagnetic waves travel at the same speed in free space (a vacuum), 3.0 × 10⁸ m/s.
- Each region has typical uses, and microwaves, infrared, ultraviolet and gamma rays are harmful in excess. Shielding, distance and limiting exposure time reduce the risk.
What is the electromagnetic spectrum? Single · Double · Physics
- Continuous means there are no gaps: one region merges into the next, and the boundaries between them are not sharp.
- Free space means a vacuum. The speed there is 3.0 × 10⁸ m/s for every region — radio waves and gamma rays arrive together. Pearson gives this value in any question that needs it.
- Light is just the small part of the spectrum that our eyes detect.
- Electromagnetic waves do not need a medium: they travel through a vacuum, which is how light and infrared from the Sun reach the Earth (taking about 500 s).
Because all the regions have the same speed, \(v=f\lambda\) means a higher frequency must go with a shorter wavelength.
What is the order of the electromagnetic spectrum? Single · Double · Physics
| Order | Region | Wavelength | Frequency |
|---|---|---|---|
| 1 | radio waves | longest | lowest |
| 2 | microwaves | ↓ decreasing | ↓ increasing |
| 3 | infrared | ||
| 4 | visible light (red → violet) | ||
| 5 | ultraviolet | ||
| 6 | x-rays | ||
| 7 | gamma rays | shortest | highest |
- A memory aid helps: Rabbits Meet In Very Unusual eXpensive Gardens (radio → gamma).
- The colours of visible light, longest wavelength first: red, orange, yellow, green, blue, indigo, violet (“ROYGBIV”).
- The names tell you the neighbours: infrared is just below red in frequency; ultraviolet is just beyond violet.
(a) Name the region of the electromagnetic spectrum between visible light and x-rays. (b) Name the colour of visible light with the highest frequency. (c) State which has the longer wavelength: microwaves or infrared.
(a) ultraviolet (b) violet (c) microwaves (they are nearer the radio end).
What are the uses of each type of electromagnetic radiation? Single · Double · Physics
“Explain” questions want the use and the property that makes it work. The middle column is what earns the second mark.
| Region | Use | Why it works |
|---|---|---|
| radio waves | broadcasting (radio and TV) and communications | easily produced and detected by aerials using alternating currents; they can carry information over long distances |
| microwaves | cooking | absorbed by water molecules in food, which gain energy, so the food heats up from within |
| microwaves | satellite transmissions (satellite TV, phones, GPS) | they pass through the atmosphere with little absorption, so signals reach satellites in space and come back |
| infrared | heaters (electric fires, grills, toasters) | infrared is absorbed by the skin and by food, transferring energy and raising their temperature |
| infrared | night-vision equipment and thermal cameras | all warm objects emit infrared; hotter objects (people, animals, engines) emit more, so they show up against a cooler background in the dark |
| visible light | optical fibres (communication, endoscopes) | light travels along the glass fibre by total internal reflection, carrying information as pulses (see 3(d)) |
| visible light | photography | the camera's sensor or film responds to the visible light focused on it, recording the image we see |
| ultraviolet | fluorescent lamps | ultraviolet produced inside the tube is absorbed by the coating on the glass, which then emits visible light |
| x-rays | seeing the internal structure of objects and materials — medical x-ray images, airport baggage scanners, checking for cracks in metal | x-rays pass through soft, low-density materials (skin, muscle, clothing) but are absorbed by dense materials (bone, metal), leaving a “shadow” image on a detector |
| gamma rays | sterilising food and medical equipment | high-energy gamma rays kill bacteria and other microorganisms; they pass through packaging, so items can be sterilised after they are sealed |
(a) Explain why gamma rays, rather than ultraviolet, are used to sterilise syringes that are already sealed in plastic packets. (b) Explain how a night-vision camera can show a fox in a dark field.
(a) Gamma rays kill bacteria (microorganisms) and they can pass through the plastic packet, so the syringe is sterilised inside the packet and stays sterile until it is opened. Ultraviolet would be absorbed by the packaging.
(b) The fox is warmer than its surroundings, so it emits more infrared. The camera detects infrared rather than visible light, so the fox shows up as a brighter image even when there is no visible light.
What are the dangers of electromagnetic radiation, and how can you protect yourself? Single · Double · Physics
Small doses of most radiations are harmless; the specification asks about excessive exposure. In general, the higher the frequency, the more energy the radiation carries and the more damage it can do to cells.
| Radiation | Harmful effect of too much exposure | Simple protective measures |
|---|---|---|
| microwaves | internal heating of body tissue (they are absorbed by water in the body) | microwave ovens have a metal case and a metal mesh in the door to keep microwaves in; do not run an oven with the door damaged or open |
| infrared | skin burns | guards around heaters and fires; heat-resistant gloves; keep a safe distance from very hot objects |
| ultraviolet | damage to surface cells (sunburn, ageing of skin, increased risk of skin cancer) and to the eyes, which can lead to blindness | sunscreen; clothing and hats; sunglasses or goggles that absorb ultraviolet; limit time in strong sunlight and avoid sunbeds |
| gamma rays | damage to cells inside the body: cancer, and mutation of DNA | lead or thick concrete shielding; increase the distance from the source; reduce the time of exposure; workers wear a badge to monitor (not reduce) their dose |
- Link each effect to where the radiation is absorbed: microwaves penetrate and heat internal tissue; infrared and ultraviolet are absorbed at the surface (skin and eyes); gamma rays pass deep into the body and damage cells there.
- X-rays have similar risks to gamma rays, which is why radiographers stand behind a lead screen and patients' doses are kept as low as possible.
A climber spends a day on a snowfield at high altitude, where ultraviolet from the Sun is strong. (a) Explain two ways in which the ultraviolet could harm the climber. (b) Describe two ways the climber can reduce the risk.
(a) Ultraviolet damages the surface cells of the skin, causing sunburn (and raising the risk of skin cancer). It also damages the eyes, which can cause blindness.
(b) Any two: wear sunglasses or goggles that block ultraviolet; apply sunscreen; cover the skin with clothing and a hat; limit the time spent in direct sunlight.
How do you do calculations with electromagnetic waves? Single · Double · Physics
Use \(v=f\lambda\) from 3(b) with \(v=3.0\times10^8\ \text{m/s}\). Convert GHz, MHz and kHz to Hz first, and use standard form on your calculator.
A microwave oven uses microwaves of frequency 2.45 GHz. An x-ray machine produces x-rays of wavelength 1.0 × 10⁻¹⁰ m. The speed of electromagnetic waves is 3.0 × 10⁸ m/s.
(a) Calculate the wavelength of the microwaves. (b) Calculate the frequency of the x-rays. (c) Use your answers to explain why x-rays are more dangerous to body cells than microwaves.
(a) \(f=2.45\times10^9\ \text{Hz}\); \(\lambda=\dfrac{v}{f}=\dfrac{3.0\times10^8}{2.45\times10^9}=0.1224=\) 0.12 m
(b) \(f=\dfrac{v}{\lambda}=\dfrac{3.0\times10^8}{1.0\times10^{-10}}=\) 3.0 × 10¹⁸ Hz
(c) The x-rays have a frequency about 10⁹ times higher (3.0 × 10¹⁸ ÷ 2.45 × 10⁹ ≈ 1.2 × 10⁹). Higher-frequency radiation carries more energy and can damage cells and DNA, causing cancer or mutation, whereas microwaves mainly cause heating.
Exam tips for 3(c)
Questions may ask for the order of increasing wavelength, which is gamma → radio — the reverse of the usual list. Read the direction in the question before you write.
For “explain a use”, one mark is the use and one is the property behind it. “Microwaves are used for cooking” earns one; adding “because water molecules in the food absorb them” earns the second.
“It is dangerous” or “it causes damage” scores nothing. Name the effect: internal heating, skin burns, damage to surface cells, blindness, cancer, mutation.
All electromagnetic waves share the same speed in a vacuum. They differ in frequency and wavelength. Never say gamma rays are faster than radio waves.
Sunscreen protects against ultraviolet, not gamma rays; lead shielding is for gamma rays and x-rays. Give a measure that would actually work for the radiation in the question.
Common misconceptions
| Misconception | Why it is wrong | Correct idea |
|---|---|---|
| Gamma rays travel faster than radio waves. | Every electromagnetic wave has the same speed in free space. | They differ only in frequency and wavelength. |
| Radio waves are sound waves. | Sound is a vibration of particles, so it needs a medium. | Radio waves are electromagnetic and can cross a vacuum; a radio turns them into sound. |
| Microwaves cook food from the outside in, like a grill. | Microwaves penetrate the food. | Water molecules throughout the outer few centimetres absorb the microwaves. |
| Infrared is only given off by very hot objects. | All objects emit some infrared. | Warmer objects emit more, which is how night vision works. |
| Violet light has the longest wavelength because it is listed last. | The list runs from long to short wavelength. | Red has the longest visible wavelength; violet the shortest. |
Key facts
| Fact | Detail | Route |
|---|---|---|
| order (long λ → short λ) | radio, microwave, infrared, visible, ultraviolet, x-ray, gamma ray | All |
| visible colours (long λ → short λ) | red, orange, yellow, green, blue, indigo, violet | All |
| speed in free space | the same for all: 3.0 × 10⁸ m/s | All |
| link to the wave equation | \(v=f\lambda\) — on the Equation List (yes — all papers); higher frequency means shorter wavelength | All |
| harmful in excess | microwave (internal heating), infrared (burns), ultraviolet (surface cells, blindness), gamma (cancer, mutation) | All |
Check your understanding
1. Write the regions of the electromagnetic spectrum in order of increasing frequency. All
radio, microwave, infrared, visible, ultraviolet, x-ray, gamma ray.
2. State one property shared by all electromagnetic waves. All
Any one: they all travel at the same speed (3.0 × 10⁸ m/s) in free space; they can all travel through a vacuum; they all transfer energy; they can all be reflected and refracted.
3. Explain why x-rays can be used to see a broken bone. All
X-rays pass through soft tissue but are absorbed by the denser bone, so the bone leaves a shadow on the detector and the break shows up as a gap.
4. Red light has a wavelength of 7.0 × 10⁻⁷ m. Calculate its frequency. (speed of light = 3.0 × 10⁸ m/s) All
\(f=\dfrac{3.0\times10^8}{7.0\times10^{-7}}=\) 4.3 × 10¹⁴ Hz
5. A long-wave radio station broadcasts at 198 kHz. Calculate the wavelength. All
\(\lambda=\dfrac{3.0\times10^8}{198\times10^3}=1515\ \text{m}\approx\) 1500 m (1.5 km).
6. A hospital worker handles gamma sources. Describe two precautions that reduce her exposure, and state the risk they protect against. All
Any two: use lead shielding; handle sources with long tongs (distance); keep handling time short. (A dose-monitoring badge records her exposure but does not reduce it.) Risk: cell damage leading to cancer or mutation.
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