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What makes up the electromagnetic spectrum, and how do sound waves travel?

Describe the electromagnetic spectrum and its uses, and explain how sound waves are produced and travel

A focused answer to the O-Level Physics outcome on the electromagnetic spectrum and sound. The order of the EM spectrum, common uses and dangers, the speed of light, and how sound is a longitudinal wave needing a medium.

Generated by Claude Opus 4.88 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
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What this dot point is asking

SEAB wants you to describe the electromagnetic spectrum, its order and common uses and dangers, to know that all electromagnetic waves travel at the speed of light in a vacuum, and to explain how sound is produced and travels as a longitudinal wave that needs a medium. The big idea is that light is one part of a large family of waves, while sound is a quite different kind of wave.

The answer

The electromagnetic spectrum

The electromagnetic (EM) spectrum is the family of transverse waves that all travel at the same speed in a vacuum, the speed of light, 3×108 m s−13 \times 10^8\ \text{m s}^{-1}. They differ in frequency and wavelength. In order of increasing frequency (and decreasing wavelength):

  1. Radio waves
  2. Microwaves
  3. Infrared
  4. Visible light
  5. Ultraviolet
  6. X-rays
  7. Gamma rays

Common uses

  • Radio waves: broadcasting radio and television.
  • Microwaves: cooking food, mobile phone and satellite communication.
  • Infrared: remote controls, thermal imaging, heating.
  • Visible light: seeing, photography, optical fibres.
  • Ultraviolet: detecting forged banknotes, sterilising, suntan.
  • X-rays: medical imaging of bones, airport security.
  • Gamma rays: killing cancer cells, sterilising equipment.

Dangers of high-frequency EM waves

The higher-frequency waves carry more energy and can damage cells. Ultraviolet can cause skin cancer and eye damage; X-rays and gamma rays can damage or kill living cells, which is why exposure is carefully limited.

Sound waves

Sound is a longitudinal wave produced by a vibrating object (such as a loudspeaker cone or vocal cords). The vibrations push and pull the surrounding particles, creating compressions and rarefactions that travel outward. Because it needs particles to pass on the vibration, sound cannot travel through a vacuum.

Sound travels at about 330 m s−1330\ \text{m s}^{-1} in air, faster in liquids, and faster still in solids, because the particles are closer together and pass on the vibration more quickly. An echo is sound reflected from a hard surface.

Examples in context

Example 1. Why we see lightning before we hear thunder. Light from the lightning reaches us almost instantly at the speed of light, while the sound of the thunder travels at only about 330 m s−1330\ \text{m s}^{-1}. Counting the seconds between the flash and the bang, then multiplying by the speed of sound, gives a rough distance to the storm.

Example 2. Communications. Mobile phones and satellites use microwaves, which pass easily through the atmosphere, while broadcast radio uses radio waves that can travel long distances and around obstacles. Choosing the right region of the EM spectrum for each job is central to all modern communication.

Try this

Q1. List the electromagnetic spectrum in order of increasing frequency. [2 marks]

  • Cue. Radio, microwave, infrared, visible light, ultraviolet, X-rays, gamma rays.

Q2. State one use of infrared radiation and one danger of X-rays. [2 marks]

  • Cue. Infrared: remote controls or heating. X-rays: can damage or kill living cells.

Q3. Explain why an astronaut on the airless Moon cannot hear a sound but can see a flash. [2 marks]

  • Cue. Sound needs particles to travel and there are none in the vacuum; light is an EM wave that travels through a vacuum, so the flash is still seen.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original5 marks(a) List the seven regions of the electromagnetic spectrum in order of increasing frequency. (b) State one use of microwaves and one danger of ultraviolet radiation.
Show worked answer →

(a) In order of increasing frequency: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays.

(b) Microwaves are used for cooking food (or for mobile phone and satellite communication). Ultraviolet radiation can damage skin cells and cause skin cancer (or damage the eyes). (Any one each.)

Markers reward the seven regions in the correct order (increasing frequency), a valid use of microwaves, and a valid danger of ultraviolet.

Original4 marks(a) Explain why sound cannot travel through a vacuum. (b) A girl claps her hands 660 m660\ \text{m} from a cliff and hears the echo 4.0 s4.0\ \text{s} later. Calculate the speed of sound in air.
Show worked answer →

(a) Sound is a longitudinal wave that needs particles to vibrate and pass on the disturbance. A vacuum has no particles, so there is nothing to carry the sound, and it cannot travel through it.

(b) The sound travels to the cliff and back, a distance of 2×660=1320 m2 \times 660 = 1320\ \text{m}, in 4.0 s4.0\ \text{s}. Speed =distancetime=13204.0=330 m s−1= \dfrac{\text{distance}}{\text{time}} = \dfrac{1320}{4.0} = 330\ \text{m s}^{-1}.

Markers reward sound needing a medium of particles (so none in a vacuum), doubling the distance for the echo's round trip, and the speed as distance over time.

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