The Electromagnetic Spectrum

Section 1 of 12The electromagnetic spectrum

• Electromagnetic (EM) waves
  • transverse waves of oscillating electric and magnetic fields that carry energy.
• Order of the spectrum
  • from long wavelength and low frequency to short wavelength and high frequency.
  • Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays.
• A memory aid
  • "Raging Martians Invade Venus Using X-ray Guns" gives the first letters in order.
• Visible light order
  • red, orange, yellow, green, blue, indigo, violet, from the longer to the shorter wavelength.
  • Violet is the highest frequency of visible light and the most energetic.

Region	Approximate wavelength	Frequency trend
Radio waves	1 mm to over 104 m	Lowest
Microwaves	1 mm to 30 cm	Low
Infrared	700 nm to 1 mm	Medium
Visible light	400 to 700 nm	Medium
Ultraviolet	10 to 400 nm	High
X-rays	0.01 to 10 nm	Very high
Gamma rays	Less than 0.01 nm	Highest

Section 2 of 12Properties common to all EM waves

• Speed in vacuum
  • all EM waves travel at 3.0 × 10⁸ m/s in a vacuum.
    • (and approximately in air)
  • This is the speed of light, written c.
• Transverse
  • the oscillations are perpendicular to the direction of travel.
• No medium needed
  • they can travel through a vacuum, which is how the light from the Sun reaches the Earth.
• They transfer energy
  • an EM wave carries energy that can be absorbed to cause heating, a chemical change or ionisation.
• They obey the wave equation
  • c = fλ, so higher frequency means shorter wavelength.
  • A wave of wavelength 3.0 m has a frequency of 3.0 × 10⁸ ÷ 3.0 = 1.0 × 10⁸ Hz.
• They can be reflected, refracted and diffracted
  • all wave properties apply.
• Energy increases with frequency
  • gamma rays carry the most energy per photon, radio waves the least.

Section 3 of 12Uses of radio waves, microwaves and infrared

1Radio waves2 points

• they are used for broadcasting radio and television, and for communication with satellites and aircraft.
• Long wavelengths diffract around hills, so they can reach receivers that are out of sight.

2Microwaves3 points

• they are used in satellite communication, mobile phones, Wi-Fi and microwave ovens.
• Satellite signals pass through the atmosphere without being absorbed.
• In an oven microwaves are absorbed by water in food, which heats it throughout.

3Infrared3 points

• it is emitted by warm objects and is used for heating, grills and toasters.
• Remote controls, intruder alarms and thermal imaging cameras use infrared.
• Infrared in optical fibres carries internet signals.

Radiation	Use	Reason
Radio	Broadcasting	Long wavelength, travels far and diffracts
Microwave	Mobile phones, satellites	Passes through the atmosphere
Microwave	Cooking	Absorbed by water molecules
Infrared	Remote control	Short range, low energy signal
Infrared	Thermal imaging	All warm objects emit it

Section 4 of 12Visible light and ultraviolet

• Visible light
  • the only part detected by the human eye, used for vision, photography and in optical fibres.
  • Plants use it for photosynthesis, and lasers use it for reading barcodes and discs.
• Ultraviolet (UV)
  • it is emitted by the Sun and by UV lamps.
  • Uses include sterilising water, detecting forged banknotes with fluorescence and security markings.
• Fluorescence
  • certain materials absorb UV and re-emit visible light, which is why UV lamps make marked items glow.
• Dangers of UV
  • it can damage skin cells and cause sunburn, skin cancer and eye damage such as cataracts.
  • Protection uses sunscreen, sunglasses and covering up.
• Ozone layer
  • the atmosphere absorbs most of the harmful UV from the Sun.

Section 5 of 12X-rays and gamma rays

• X-rays
  • they pass through soft tissue but are absorbed by bone and denser materials.
  • Used in medical imaging for broken bones, in dental checks and in airport baggage scanners.
  • Photographic film or detectors record the shadow image.
• Gamma rays
  • they are the most penetrating EM waves, emitted by radioactive nuclei.
  • Used to kill cancer cells in radiotherapy, to sterilise surgical instruments and food, and to detect leaks in pipes.
• Dangers
  • X-rays and gamma rays are ionising radiation that can damage or kill living cells and cause mutations leading to cancer.
  • Workers wear badges to monitor exposure and use lead shielding and short exposure times.
• Thick lead or concrete
  • it is needed to absorb gamma rays, while X-rays can be blocked by thinner lead sheets.

Section 6 of 12Hazards and safe use of EM radiation

Radiation	Hazard	Precaution
Microwaves	Internal heating of body cells	Shielded ovens with metal mesh
Infrared	Skin burns from too much heat	Avoid prolonged exposure
Ultraviolet	Skin cancer, eye damage	Sunscreen, goggles
X-rays	Ionisation, cancer	Lead aprons, minimum exposure
Gamma rays	Ionisation, cell damage	Lead shielding, distance, short times

• Ionising radiation
  • it has enough energy to remove electrons from atoms, so it can damage DNA.
• Non-ionising radiation
  • radio waves, microwaves and infrared are non-ionising but may heat tissue.
  • Visible light and low-energy UV usually do not ionise, but they can still cause harm.
• Mobile phone use
  • the evidence for harm is limited, but the radiation can heat tissues slightly.
• Choosing a wave for a purpose
  • consider whether it must travel through walls, the atmosphere or tissue, and how much energy it carries.

Section 7 of 12Communication using EM waves

• Analogue signals
  • they vary continuously in amplitude or frequency and are affected by noise as they travel.
• Digital signals
  • they use a series of pulses representing 1 and 0 and can be regenerated.
  • Digital signals carry more information with less noise and give better quality.
• Transmission methods
  • radio and microwave signals pass through the air or space, while light and infrared signals pass along fibres.
• Satellite communication
  • signals are sent to a satellite on a microwave beam, amplified and re-transmitted.

Section 8 of 12Applications of each region

Region	Applications
Radio waves	Radio and television communication, astronomy
Microwaves	Satellite television, mobile (cell) phones, Bluetooth, microwave ovens
Infrared	Household electrical appliances, remote controllers, intruder alarms, thermal imaging, optical fibres
Visible light	Photography, vision
Ultraviolet	Security marking, detecting counterfeit bank notes, sterilising water
X-rays	Medical imaging, security scanners, killing cancerous cells, finding cracks in metal in engineering
Gamma rays	Detecting and killing cancer, sterilising food and medical equipment, finding cracks in metal

• Radio astronomy
  • radio telescopes collect radio waves from stars and galaxies, which can pass through dust clouds that block light.
• Bluetooth and mobile phones
  • they use microwaves to carry signals over short and long distances respectively.
• Security marking
  • an invisible ink shows up under ultraviolet light, so marked property can be identified.
• Sterilising water
  • ultraviolet light kills bacteria in water without adding chemicals.
• Engineering with X-rays and gamma rays
  • radiation passes through metal and darkens film, and a crack shows up where more radiation gets through.

Section 9 of 12Damage caused by electromagnetic radiation

• Heating and burns
  • too much exposure to microwaves or infrared heats soft tissues and can cause burns.
  • Standing too close to a powerful heater or touching a microwave oven's open door seal with the oven on are examples to avoid.
• Ionising effects of ultraviolet
  • it can cause skin cancer and cataracts of the eye.
• Ionising effects of X-rays and gamma rays
  • they can cause cell mutation and cancer, and kill cells.
• Reducing exposure
  • use shielding, protective clothing, sunscreen, tinted lenses and short exposure times.

Section 10 of 12Worked examples

1. Put these in order of increasing frequency: X-rays, radio waves, visible light, infrared, gamma rays. [2]2 marks

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2. Calculate the frequency of ultraviolet light with a wavelength of 2.0 × 10^-7 m (c = 3.0 × 10⁸ m/s). [3]3 marks

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3. A radio wave takes 1.3 s to travel from the Earth to the Moon. Calculate the distance. [3]3 marks

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4. State one use and one danger of gamma rays. [2]2 marks

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5. Explain why microwaves are used for satellite communication but not X-rays. [2]2 marks

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6. Give one use and one hazard for each of microwaves and X-rays. [4]4 marks

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Section 11 of 12Key terms

Term	Meaning
Electromagnetic wave	A transverse wave of electric and magnetic fields that travels at 3.0 × 108 m/s in a vacuum.
Electromagnetic spectrum	The family of EM waves arranged by wavelength and frequency.
Ionising radiation	Radiation with enough energy to remove electrons from atoms.
Infrared	EM radiation emitted by warm objects, with wavelength longer than visible red.
Ultraviolet	EM radiation with wavelength shorter than violet light.
Gamma ray	A very penetrating EM wave emitted by atomic nuclei.
Fluorescence	Absorption of UV that is re-emitted as visible light.
Digital signal	A signal made of pulses that represent 1 and 0.

Section 12 of 12Exam technique

1. Saying all EM waves have the same frequency — they share the same speed in vacuum but have different frequencies.
2. Thinking microwaves cook from the inside only — they are absorbed in the outer centimetres and conduction heats the rest.
3. Confusing ultraviolet and infrared — UV has the shorter wavelength and the higher energy.
4. Saying sound is an electromagnetic wave — sound is a longitudinal mechanical wave.
5. Saying all radiation is ionising — only UV at the high end, X-rays and gamma rays have enough energy.