P23: Electromagnetic Spectrum
The electromagnetic spectrum in order from radio waves to gamma rays, the properties of EM waves, and the uses and dangers of each type of radiation.
Properties of Electromagnetic Waves
All electromagnetic (EM) waves are transverse waves that transfer energy from a source to an absorber. They all travel at the same speed through a vacuum: approximately 3 × 10⁸ m/s.
- They are transverse — oscillations are perpendicular to the direction of energy transfer
- They can travel through a vacuum (no medium needed)
- They travel at 3 × 10⁸ m/s in a vacuum
- They can be reflected, refracted and diffracted
- They can be absorbed and emitted by objects
The EM spectrum is a continuous range of wavelengths and frequencies. Different types of EM radiation are grouped based on their wavelength or frequency, but there are no sharp boundaries between groups.
EM waves are produced by changes in atoms or their nuclei. For example, changes in the nucleus produce gamma radiation, and changes in electron energy levels produce UV, visible light and infrared.
The EM Spectrum in Order
From longest wavelength and lowest frequency to shortest wavelength and highest frequency:
Radio → Microwave → Infrared → Visible light → Ultraviolet → X-ray → Gamma
| Type | Typical Wavelength | Frequency | Energy | Danger |
|---|---|---|---|---|
| Radio waves | > 1 m | Lowest | Lowest | Lowest |
| Microwaves | 1 mm – 1 m | Low | Low | Low |
| Infrared | 700 nm – 1 mm | Medium-low | Medium-low | Low |
| Visible light | 400 – 700 nm | Medium | Medium | Low |
| Ultraviolet | 10 – 400 nm | Medium-high | Medium-high | High |
| X-rays | 0.01 – 10 nm | High | High | High |
| Gamma rays | < 0.01 nm | Highest | Highest | Highest |
Mnemonic: Read My Incredible Visible Unique X-ray Glasses (Radio, Microwave, Infrared, Visible, UV, X-ray, Gamma).
As you move across the spectrum from radio to gamma: wavelength decreases, frequency increases, and energy increases. These are all linked because v = f × λ, and all EM waves travel at the same speed in a vacuum.
Uses of EM Waves
Radio Waves
- Television and radio broadcasting
- Long-wave radio reflects off the ionosphere for long-distance communication
- Bluetooth and short-range communication
Microwaves
- Cooking — microwaves are absorbed by water molecules, heating food
- Satellite communication — microwaves pass through the atmosphere without significant absorption
- Mobile phone networks and Wi-Fi
Infrared
- Heating — infrared heaters, grills, toasters
- Thermal imaging cameras for medicine, security and building surveys
- Remote controls and optical fibre communication
Visible Light
- Human vision and illumination
- Photography and fibre optic communication
Ultraviolet
- Sunbeds and fluorescent lamps
- Security markings — fluorescent inks visible under UV light
- Sterilising water and surgical equipment — UV kills bacteria
X-rays
- Medical imaging — X-rays pass through soft tissue but are absorbed by bone
- Security scanners at airports
- Industrial inspection of metal components for cracks
Gamma Rays
- Sterilising medical equipment and food
- Cancer treatment (radiotherapy) — targeted gamma rays kill cancer cells
- Medical tracers — radioactive sources injected to diagnose conditions
Dangers of EM Waves
Higher frequency EM waves (UV, X-rays and gamma rays) are ionising radiation. They have enough energy to remove electrons from atoms, which can damage DNA and cause cancer.
| EM Wave | Hazard | Precautions |
|---|---|---|
| Radio waves | Minimal risk at normal exposure | No special precautions needed |
| Microwaves | Internal heating of body tissue | Keep away from operating ovens; shielding on devices |
| Infrared | Skin burns from thermal radiation | Protective clothing; avoid intense sources |
| Visible light | Eye damage from intense light or lasers | Avoid looking at bright sources; wear sunglasses |
| Ultraviolet | Skin burns, premature ageing, skin cancer, eye damage | Wear sunscreen, UV-protective clothing, sunglasses |
| X-rays | Ionising — mutation, cancer, gene damage | Lead shielding; limit exposure; wear dosimeter badges |
| Gamma rays | Ionising — mutation, cancer, cell death | Lead or thick concrete shielding; minimise exposure time; increase distance |
Ionising radiation (UV, X-rays, gamma) can damage DNA and cause cancer. Non-ionising radiation (radio, micro, IR, visible) does not have enough energy to ionise atoms but can still cause heating effects.
Generating and Detecting EM Waves
Radio waves are produced by oscillating charges in an AC circuit in a transmitter. When radio waves are absorbed by a receiver, they can create an alternating current with the same frequency as the radio wave.
- Changing the frequency of oscillation changes the frequency of the radio waves produced
- UV, X-rays and gamma rays can be generated by changes in atoms or atomic nuclei
- Changes in the nucleus produce gamma radiation
- Changes in electron energy levels produce UV and X-ray radiation
Radio waves can be used for communication because they can travel long distances. Long-wavelength radio waves are reflected by the ionosphere, allowing them to reach receivers beyond the horizon. Short-wavelength radio waves (and microwaves) pass through the ionosphere, making them suitable for satellite communication.
Practice Questions
1. Name the seven types of electromagnetic radiation in order of increasing frequency. [2 marks]
Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays.
2. Explain why X-rays and gamma rays are more dangerous than radio waves. [3 marks]
X-rays and gamma rays are ionising radiation with high frequency and high energy. They can remove electrons from atoms, damage DNA and mutate cells, potentially causing cancer. Radio waves are non-ionising and have much lower energy.
3. State two uses of infrared radiation and two uses of ultraviolet radiation. [4 marks]
Infrared: heaters/cooking, remote controls, thermal imaging. Ultraviolet: sunbeds, security markings, sterilising equipment.
4. Explain why microwaves are used for satellite communication rather than radio waves. [2 marks]
Microwaves can pass through the Earth's atmosphere without being significantly absorbed or reflected, unlike long-wavelength radio waves which are reflected by the ionosphere.
5. Describe the relationship between wavelength, frequency, and energy across the EM spectrum. [3 marks]
As wavelength decreases, frequency and energy increase. Gamma rays have the shortest wavelength, highest frequency and highest energy. Radio waves have the longest wavelength, lowest frequency and lowest energy.
6. Describe how radio waves are produced and detected. [3 marks]
Radio waves are produced by oscillating charges in an AC circuit in a transmitter. When the radio waves reach a receiver, they are absorbed and create an alternating current in the aerial with the same frequency as the original radio wave.
Maths Skills
Standard Form for EM Spectrum Ranges
The EM spectrum covers an enormous range of wavelengths and frequencies. Standard form is essential for expressing these values clearly.
Wavelengths in Standard Form
- Radio waves: wavelength up to 10⁶ m (1,000,000 m)
- Microwaves: 1 mm to 1 m — i.e. 10⁻³ m to 10⁰ m
- Infrared: 700 nm to 1 mm — i.e. 7 × 10⁻⁷ m to 10⁻³ m
- Visible light: 400–700 nm — i.e. 4 × 10⁻⁷ m to 7 × 10⁻⁷ m
- Ultraviolet: 10–400 nm — i.e. 10⁻⁸ m to 4 × 10⁻⁷ m
- X-rays: 0.01–10 nm — i.e. 10⁻¹¹ m to 10⁻⁸ m
- Gamma rays: less than 0.01 nm — i.e. < 10⁻¹¹ m
Comparing Wavelengths
Worked Example
How many times longer is a typical radio wave (100 m) than a typical X-ray wavelength (0.1 nm)?
Ratio = 100 / (0.1 × 10⁻⁹) = 100 / 10⁻¹⁰ = 10² / 10⁻¹⁰ = 10¹² = 1,000,000,000,000 times longer.
Energy and Frequency Relationships
- Since v = fλ and v is constant for all EM waves in a vacuum, f and λ are inversely proportional.
- As you move from radio to gamma: frequency increases by a factor of about 10¹⁸ and wavelength decreases by the same factor.
- Energy of a photon is directly proportional to frequency: E = hf (Higher tier). This means gamma ray photons carry far more energy than radio wave photons.
- A doubling of frequency means a halving of wavelength (and vice versa).
Using v = fλ Across the Spectrum
A microwave has a frequency of 10 GHz (10 × 10⁹ Hz). Calculate its wavelength.
λ = v / f = 3 × 10⁸ / 1 × 10¹⁰ = 3 × 10⁻² m = 0.03 m = 3 cm.
Common Misconceptions
"All radiation is dangerous." — Only ionising radiation is particularly harmful. Non-ionising radiation (radio waves, microwaves, infrared, visible light) does not have enough energy per photon to remove electrons from atoms or damage DNA directly. The main risk from non-ionising radiation is heating. Ionising radiation (UV, X-rays, gamma rays) can damage DNA and cause cancer because each photon carries enough energy to ionise atoms.
"Radio waves are sound waves." — Radio waves are electromagnetic waves, not sound waves. Radio waves travel at the speed of light (3 × 10⁸ m/s) and can travel through a vacuum. Sound waves are longitudinal mechanical waves that require a medium and travel at about 340 m/s in air. A radio receives EM radio waves and converts them into electrical signals that drive a speaker to produce sound waves.
"Gamma rays and X-rays are completely different." Gamma rays and X-rays overlap in frequency and wavelength. The distinction is their origin: gamma rays are produced by changes in the nucleus, while X-rays are produced by changes in electron energy levels. They can have the same frequency and the same ionising effect on tissue.
6-Mark Extended Question
Explain why different parts of the electromagnetic spectrum are used for different communications applications. Discuss the risks associated with ionising radiation. [6 marks]
Radio waves are used for broadcasting because their long wavelengths can diffract around hills and buildings, and long-wave radio reflects off the ionosphere to reach receivers beyond the horizon. Microwaves are used for satellite communication because they pass through the atmosphere without significant absorption or reflection, unlike long-wavelength radio waves which are reflected by the ionosphere. Infrared is used in optical fibre communication because it can carry more information per second than radio waves and is totally internally reflected along the fibre. Visible light is also used in fibre optics for high-speed data transfer. The ionising part of the spectrum (UV, X-rays, gamma rays) is not used for routine communication because these waves carry enough energy per photon to ionise atoms and damage DNA, increasing the risk of cancer and mutations. Precautions such as lead shielding, limiting exposure time, and wearing dosimeter badges are needed when working with ionising radiation. Non-ionising radiation is safer for communication as it only causes heating effects rather than DNA damage.
AO3: Analyse and Evaluate
The table below shows data about different EM wave applications:
| Application | EM Wave Type | Typical Power | Distance from Source | Safety Measure |
|---|---|---|---|---|
| Mobile phone mast | Microwave | 20 W | 5 m (public) | Restriction zones |
| Wi-Fi router | Microwave (2.4 GHz) | 0.1 W | 1 m (user) | None required |
| Medical X-ray | X-ray | 1000 W (pulsed) | 0.5 m (patient) | Lead apron, limited exposure |
| Radiotherapy | Gamma | 500 W | 0.3 m (target) | Shielded room, precise targeting |
(a) Explain why X-rays and gamma rays require more safety precautions than microwaves despite the mobile phone mast having comparable power.
(b) A Wi-Fi router emits 0.1 W of microwave radiation at 2.4 GHz. A mobile phone mast emits 20 W at 1.8 GHz. Evaluate which poses a greater risk to a person standing 5 m from the mast compared to 1 m from the router, considering both the power and the type of radiation.
(a) X-rays and gamma rays are ionising radiation: each photon carries enough energy to remove electrons from atoms and damage DNA, potentially causing cancer. The risk depends on the energy per photon, not just the total power. Microwaves are non-ionising and can only cause heating effects. Even at comparable power levels, ionising radiation is far more dangerous per photon because it can cause irreversible cellular damage.
(b) Both sources emit non-ionising microwaves, so the main risk is heating. Intensity decreases with the square of distance (inverse square law). For the mast at 5 m: intensity ∝ 20 / 5² = 20 / 25 = 0.8 W/m². For the router at 1 m: intensity ∝ 0.1 / 1² = 0.1 W/m². The mast delivers about 8 times greater intensity at the given distances. However, the router is used continuously in close proximity while masts are only briefly near people. Both are well below safety limits for non-ionising radiation. The mast may pose a marginally higher risk at the specified distances, but both are considered safe at these power levels.
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