P22: Electromagnetic Spectrum
The EM spectrum, properties and uses
Electromagnetic Spectrum
๐ Key Definitions
Electromagnetic waves: Transverse waves that transfer energy from the source to the absorber. They can all travel through a vacuum at the same speed (3 × 10⁸ m/s). They are produced by changes in atoms and nuclei.
Ionising radiation: Radiation that can damage or destroy cells by removing electrons from atoms. Higher frequency EM waves (UV, X-rays, gamma rays) are ionising.
๐ The EM Spectrum in Order
All EM waves travel at the speed of light in a vacuum: 3 × 10⁸ m/s. From longest wavelength/lowest frequency to shortest wavelength/highest frequency:
Radio → Microwave → Infrared → Visible light → Ultraviolet → X-rays → Gamma rays
< Longest wavelength / Lowest frequency | Shortest wavelength / Highest frequency >
< Least energy | Most energy >
< Least dangerous | Most dangerous >
< Longest wavelength / Lowest frequency | Shortest wavelength / Highest frequency >
< Least energy | Most energy >
< Least dangerous | Most dangerous >
| Wave type | Wavelength range | Source | Detection |
|---|---|---|---|
| Radio waves | > 1 m | Alternating currents in circuits | Aerials, radio receivers |
| Microwaves | 1 mm – 1 m | Microwave ovens, satellites | Microwave receivers |
| Infrared | 700 nm – 1 mm | Hot objects, flames | Infrared cameras, skin |
| Visible light | 400 – 700 nm | Sun, lamps, lasers | Eyes, photographic film |
| Ultraviolet | 10 – 400 nm | Sun, UV lamps | Fluorescent materials |
| X-rays | 0.01 – 10 nm | X-ray tubes, stars | Photographic film, digital sensors |
| Gamma rays | < 0.01 nm | Radioactive decay, nuclear reactions | Geiger-Muller tube |
Key relationships: As you go from radio to gamma: wavelength decreases, frequency increases, energy increases, danger increases. Since v = fλ and v is constant (3 × 10⁸ m/s in vacuum), if frequency increases then wavelength must decrease.
๐ Uses and Dangers
| Wave type | Uses | Dangers |
|---|---|---|
| Radio waves | Broadcasting (TV, radio), communication, Bluetooth | Very low risk — not ionising |
| Microwaves | Cooking (microwave ovens), satellite communication, Wi-Fi | Internal heating of body tissues |
| Infrared | Heaters, remote controls, night vision cameras, optical fibres | Skin burns from intense IR |
| Visible light | Fibre optic communication, vision, photography | Very bright light can damage eyes |
| Ultraviolet | Fluorescent lamps, security markings, tanning beds | Damage to skin and eyes, premature skin ageing, skin cancer |
| X-rays | Medical imaging (bones), airport security scanners | Ionising — can damage DNA, cause cancer |
| Gamma rays | Sterilising medical equipment, cancer treatment (radiotherapy), sterilising food | Ionising — highly damaging to cells, mutations, cancer |
Danger principle: The higher the frequency of the EM radiation, the more dangerous it is. UV, X-rays and gamma rays are ionising — they can damage cells and DNA, potentially causing cancer. The risk increases with the dose of radiation received.
๐งฎ Worked Examples
Example 1: Calculating wavelength from frequency
A radio station broadcasts at a frequency of 100 MHz. Calculate the wavelength of the radio wave.
Solution:
f = 100 MHz = 100 × 10⁶ Hz = 1 × 10⁸ Hz
v = f × λ, so λ = v ÷ f = 3 × 10⁸ ÷ 1 × 10⁸ = 3 m
Example 2: Calculating frequency from wavelength
Microwaves used in cooking have a wavelength of 12 cm. Calculate their frequency.
Solution:
λ = 12 cm = 0.12 m
f = v ÷ λ = 3 × 10⁸ ÷ 0.12 = 2.5 × 10⁹ Hz (or 2.5 GHz)
Example 3: Comparing EM waves
Explain why gamma rays are more dangerous than radio waves.
Solution:
Gamma rays have a much higher frequency than radio waves. Higher frequency EM radiation carries more energy and is ionising — it can remove electrons from atoms and damage DNA in cells, which can cause cancer. Radio waves are non-ionising so they cannot damage cells in this way.
Example 4: Time for EM wave to travel
Light from the Sun takes 8.3 minutes to reach Earth. Calculate the distance from the Sun to the Earth.
Solution:
t = 8.3 min = 8.3 × 60 = 498 s
distance = speed × time = 3 × 10⁸ × 498 = 1.49 × 10¹¹ m (about 150 million km)
โ Practice Questions
Q1: Foundation Name the seven types of EM radiation in order of increasing frequency.
Q2: Foundation State two uses of infrared radiation and one danger.
Q3: Higher A microwave has a wavelength of 6 cm. Calculate its frequency.
Q4: Higher Explain why X-rays are suitable for medical imaging but gamma rays are used for cancer treatment instead.
Q5: Foundation State the speed of all EM waves in a vacuum.
โ Answers
- Q1: Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays.
- Q2: Uses: heaters, remote controls, night vision cameras (any two). Danger: skin burns from intense infrared radiation.
- Q3: λ = 6 cm = 0.06 m. f = v ÷ λ = 3 × 10⁸ ÷ 0.06 = 5 × 10⁹ Hz (5 GHz)
- Q4: X-rays can penetrate soft tissue but are absorbed by bone, producing images. Gamma rays are more penetrating and ionising — they can kill cancer cells but also damage healthy cells, so they are used in controlled doses for radiotherapy where cell destruction is the aim.
- Q5: 3 × 10⁸ m/s (speed of light in a vacuum).
๐ฏ Exam Tips
- Learn the EM spectrum order using a mnemonic: Read Many Instructions Very Usefully Xcept Gambling (Radio, Microwave, Infrared, Visible, UV, X-ray, Gamma).
- Remember: all EM waves are transverse and can travel through a vacuum — this is a very common exam point.
- When asked about dangers, always link higher frequency to ionisation and cell/DNA damage.
- UV is the first ionising radiation in the spectrum — this is a common trick question.
- For calculation questions, remember v = fλ always uses the speed of light (3 × 10⁸ m/s) for EM waves in a vacuum.
- Be precise with uses: "cooking" for microwaves, "medical imaging" for X-rays, "sterilisation" for gamma rays.
๐ข Maths Skills
Mathematical Skills
Using v = f ร ฮป with the speed of light (3 ร 10โธ m/s) to calculate EM wave frequencies or wavelengths. Standard form is essential for EM calculations since frequencies range from kHz (10ยณ) to EHz (10ยนโธ). Convert wavelength units before calculating: nm โ m (ร10โปโน), mm โ m (ร10โปยณ).
Maths Example
A mobile phone uses microwaves with a frequency of 1.8 GHz. Calculate the wavelength. f = 1.8 GHz = 1.8 ร 10โน Hz. ฮป = v รท f = 3 ร 10โธ รท 1.8 ร 10โน = 0.167 m = 16.7 cm.
โ ๏ธ Common Misconceptions
Watch Out!
1. Wrong: All EM waves travel at different speeds Correct: All EM waves travel at the same speed in a vacuum (3 ร 10โธ m/s) โ they differ in frequency and wavelength only
2. Wrong: Visible light is the only EM radiation that is harmful Correct: UV, X-rays and gamma rays are ionising and can damage DNA โ visible light and radio waves are non-ionising and much less dangerous
3. Wrong: Microwaves are only used for cooking Correct: Microwaves are also used for satellite communication, Wi-Fi, and mobile phone signals
โ๏ธ 6-Mark Question
Extended Answer
6 marks: Compare the dangers of different types of EM radiation. Explain why some types are more dangerous than others and give an example of a use for each of the three most dangerous types.
EM radiation becomes more dangerous as frequency increases. Radio waves and microwaves are low frequency and non-ionising โ microwaves can cause internal heating of tissues but are not generally dangerous at low doses. Infrared and visible light are also non-ionising, though intense IR can cause skin burns. UV, X-rays and gamma rays are high frequency and ionising โ they carry enough energy to remove electrons from atoms, damaging cells and DNA, which can cause mutations and cancer. UV is used in fluorescent lamps and tanning beds but can cause skin cancer and eye damage. X-rays are used in medical imaging (bone fractures) but exposure must be limited as they can damage DNA. Gamma rays are used in cancer treatment (radiotherapy) and sterilisation, but are the most penetrating and ionising, requiring heavy shielding.
Mark scheme: 1 mark โ state that danger increases with frequency, 1 mark โ explain ionisation (removing electrons/damaging DNA), 1 mark โ identify UV, X-rays and gamma as ionising, 1 mark โ use of UV (fluorescent lamps/tanning), 1 mark โ use of X-rays (medical imaging), 1 mark โ use of gamma rays (sterilisation/radiotherapy)
๐ AO3: Analyse & Evaluate
Analysis and Evaluation
The table shows the frequency and typical use of four EM waves: Radio (100 MHz โ broadcasting), Microwave (2.45 GHz โ oven), Infrared (3 ร 10ยนยณ Hz โ remote control), UV (3 ร 10ยนโต Hz โ tanning lamp).
(a) Calculate the wavelength of the microwave used in the oven.
(b) A student says "the microwave oven radiation must be dangerous because it is called radiation." Evaluate this statement.
(c) The UV tanning lamp has a wavelength of 100 nm. A phone charger emits radio waves at 50 kHz. Which has the longer wavelength? Show your working.
Answers: (a) ฮป = v/f = 3 ร 10โธ รท 2.45 ร 10โน = 0.122 m = 12.2 cm. (b) The statement is misleading โ microwave radiation is non-ionising. It heats water molecules in food but does not damage DNA like ionising radiation (UV, X-rays, gamma). However, direct exposure to intense microwaves could cause internal tissue heating, so the oven is shielded. The word "radiation" alone does not mean dangerous. (c) UV lamp: ฮป = 100 nm = 1 ร 10โปโท m. Phone charger: ฮป = v/f = 3 ร 10โธ รท 50,000 = 6000 m. The phone charger radio waves have a much longer wavelength (6000 m vs 0.0000001 m).
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