3.3 Electromagnetic spectrum
- Syllabus
- 0625–2026–2027
- Topic
- 3.3
- Level
- —
Learning objectives
3.3.1Main regions of the EM spectrum in• Know the main regions of the electromagnetic spectrum in order of frequency and in order of wavelength3.3.2All EM waves travel at the same high• Know: all electromagnetic waves travel at the same high speed in a vacuum3.3.3EM spectrum uses: (a) radio waves• Describe typical EM spectrum uses: (a) radio waves: radio/TV, astronomy, RFID (b) microwaves: satellite TV, mobile phones, microwave ovens (c) infrared: grills, remotes, intruder alarms, thermal imaging, optical fibres (d) visible light: vision, photography, illumination (e) ultraviolet: security marking, fake banknote detection, sterilising water (f) X-rays: medical scans, security scanners (g) gamma rays: sterilising food/medical equipment, cancer detection/treatment3.3.4Harm from excessive EM radiation: (a)• Describe harm from excessive EM radiation: (a) microwaves: internal body-cell heating (b) infrared: skin burns (c) ultraviolet: surface-cell/eye damage, skin cancer and eye conditions (d) X-rays/gamma rays: mutation or cell damage3.3.5Communication with artificial• Know: communication with artificial satellites is mainly by microwaves: (a) some satellite phones use low orbit artificial satellites (b) some satellite phones and direct broadcast satellite television use geostationary satellites3.3.6Speed of EM waves in a vacuum is 3.0 ו Know: the speed of electromagnetic waves in a vacuum is 3.0 × 108 m/s and is approximately the same in air3.3.7EM radiation: (a) mobile• Know communication systems using EM radiation: (a) mobile phones/wireless internet use microwaves because they penetrate some walls and need short aerials (b) Bluetooth uses radio waves because they pass through walls but weaken (c) optical fibres use visible/infrared for cable TV and high-speed broadband because glass transmits them and they carry high data rates3.3.8Difference between a digital and• Know the difference between a digital and analogue signal3.3.9A sound can be transmitted as a• Know: a sound can be transmitted as a digital or analogue signal3.3.10Benefits of digital signalling• Explain the benefits of digital signalling including increased rate of transmission of data and increased range due to accurate signal regeneration
Order the electromagnetic spectrum
All electromagnetic waves are transverse waves. They form one continuous spectrum divided into named regions by frequency and wavelength.
| Direction | Regions |
|---|---|
| increasing frequency | radio waves → microwaves → infrared → visible light → ultraviolet → X-rays → gamma rays |
| increasing wavelength | gamma rays → X-rays → ultraviolet → visible light → infrared → microwaves → radio waves |
In a vacuum, wave speed is common to every region and equals frequency multiplied by wavelength. Therefore higher frequency means shorter wavelength.
The spectrum order concerns frequency or wavelength, not wave speed. Radio waves have the longest wavelengths and lowest frequencies; gamma rays have the shortest wavelengths and highest frequencies.
Know that every EM wave has the same vacuum speed
Every region of the electromagnetic spectrum travels at the same high speed in a vacuum.
Radio waves, infrared, visible light, ultraviolet, X-rays and gamma rays sent together through the same vacuum cover the same distance in the same time, even though their frequencies and wavelengths differ.
| Property across EM regions in vacuum | Same or different? |
|---|---|
| speed | same |
| frequency | different |
| wavelength | different and inversely ordered to frequency |
Do not transfer this rule to sound or ultrasound: mechanical waves require a medium and travel far more slowly. EM waves can travel through empty space.
Match EM regions to their typical uses
A region is chosen because its interaction with matter, transmission through materials or atmosphere, detectability and energy suit the task.
| Region | Typical uses required here |
|---|---|
| radio waves | radio and television transmission; astronomy; RFID |
| microwaves | satellite television; mobile phones; microwave ovens |
| infrared | electric grills; television remotes; intruder alarms; thermal imaging; optical fibres |
| visible light | vision; photography; illumination |
| ultraviolet | security marking; detecting fake banknotes; sterilising water |
| X-rays | medical scanning; security scanners |
| gamma rays | sterilising food and medical equipment; detecting and treating cancer |
Use the exact region–application pair before explaining it: for example, infrared reveals temperature patterns in thermal imaging, X-rays penetrate soft tissue more than bone, and gamma radiation can destroy living cells and microorganisms.
Do not swap satellite television to radio waves or cancer treatment to X-rays in this syllabus mapping: satellite links use microwaves, while gamma rays are specified for cancer detection/treatment and sterilisation.
Link excessive EM exposure to harm
Harm depends on absorbed energy and dose: greater intensity, longer exposure or repeated exposure increases risk.
| Radiation | Harm from excessive exposure |
|---|---|
| microwaves | internal heating of body cells |
| infrared | skin burns |
| ultraviolet | damage to surface cells and eyes; skin cancer and eye conditions |
| X-rays and gamma rays | mutation or damage to cells |
Risk is reduced by limiting exposure time, increasing distance where appropriate, using shielding and avoiding unnecessary exposure. Medical use balances a controlled dose against its benefit.
A useful application does not make radiation harmless. The specified hazard follows the radiation and absorbed dose, not whether the source is labelled medical, domestic or industrial.
Distinguish low-orbit and geostationary satellite links
Communication with artificial satellites is mainly by microwaves, which can pass through the atmosphere to and from an aerial or dish.
| Satellite arrangement | Syllabus communication example | Key feature |
|---|---|---|
| low orbit | some satellite phones | satellite is relatively close but moves across the sky; a network or handover can maintain coverage |
| geostationary orbit | some satellite phones and direct-broadcast satellite television | satellite stays above the same point on the equator, so a fixed dish can keep pointing at it |
A ground transmitter sends a microwave uplink to the satellite; the satellite relays a downlink towards the receiving region. The journey still takes time because the signal covers a large distance.
Geostationary does not mean stationary in space: the satellite orbits once per day in the same direction as Earth rotates, so it appears fixed from the ground.
Use the numerical speed of EM waves
The speed of every electromagnetic wave in a vacuum is 3.0 × 10^8 m/s. Its speed in air is approximately the same.
| Equivalent form | Value |
|---|---|
| metres per second | 300 000 000 m/s |
| kilometres per second | 300 000 km/s |
For one-way travel, distance = speed × time. For a reflected signal that returns to its starting point, the measured time covers twice the one-way distance, so distance to the reflector = speed × total time ÷ 2.
Do not use 340 m/s: that is approximately the speed of sound in air. Also keep distance units consistent with m/s before multiplying or dividing.
Choose EM radiation for communication systems
| System | Radiation | Why it suits the system |
|---|---|---|
| mobile phones and wireless internet | microwaves | penetrate some walls and require only short aerials for transmission and reception |
| Bluetooth | radio waves | pass through walls, although the signal weakens as it travels |
| optical-fibre cable television and high-speed broadband | visible light or short-wavelength infrared | glass transmits these waves and they can carry high data rates |
A communication system encodes information at a transmitter, sends it on an electromagnetic carrier through air or glass, and detects and decodes it at the receiver.
Match all three parts of an explanation: named system, correct EM region, and the specific transmission or aerial property that makes it useful.
Optical fibre does not use microwaves in this syllabus, and Bluetooth is assigned to radio waves. Signal weakening through walls explains Bluetooth's limited useful range; it does not mean radio waves cannot pass through walls.
Distinguish analogue and digital signals
An analogue signal varies continuously and can take any value within a range.
A digital signal uses discrete levels; in the binary case it has two allowed states, commonly labelled 0 and 1 or low and high.
| Feature | Analogue | Digital |
|---|---|---|
| allowed values | continuous range | two discrete binary states |
| typical trace | smoothly varying level | sequence of high and low levels |
| small added disturbance | directly changes the represented level | can often be rejected if each level remains recognisable |
Digital does not mean the physical voltage changes instantaneously or that the carrier is always on/off. It means the information is represented by discrete states rather than a continuous range.
Transmit sound as analogue or digital information
A microphone converts changing air pressure from a sound into a changing electrical signal.
| Route | Representation of the sound |
|---|---|
| analogue transmission | the continuously varying signal follows the sound waveform and is carried or modulated for transmission |
| digital transmission | the analogue signal is sampled, each sample is encoded as numbers/binary data, and the bit sequence is transmitted |
At the receiver, an analogue signal can drive a loudspeaker after processing. Digital data is decoded and converted back to an analogue electrical waveform before the loudspeaker recreates the sound.
The original sound wave in air is not itself digital. Digital describes how information about the sound is represented and transmitted after conversion and encoding.
Explain why digital signalling extends rate and range
Digital signalling supports a high rate of data transmission and a long useful range because discrete states can be regenerated accurately.
| Stage | What happens | Benefit |
|---|---|---|
| encode and transmit bits | information is carried as sequences of discrete states | large data streams can be processed, compressed and sent at high rates |
| noise and attenuation affect the link | received pulses become weaker or distorted | states can still be identified while they remain above decision margins |
| repeater/regenerator decides 0 or 1 | outputs clean standard pulses rather than simply amplifying the distorted waveform | noise does not accumulate in the same way, increasing reliable range |
An analogue amplifier boosts both the wanted waveform and superposed noise. A digital regenerator reconstructs the intended discrete levels, provided the corruption has not made them ambiguous.
Digital transmission is not immune to noise and does not guarantee perfect reception. Its range advantage depends on sufficiently accurate detection and regeneration before errors become too large.