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6.2.1Waves

Electromagnetic (EM) Waves — AQA GCSE Combined Science Trilogy 8464 - Physics Revision Notes & Topical Questions

5 min read 9 parts Lesson 7 of 15

📚 AQA GCSE Combined Science Trilogy 8464 - Physics Hub / Topical Revision
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Part 1 of 7The electromagnetic spectrum

Electromagnetic (EM) waves form a continuous family of transverse waves, all of which are oscillating electric and magnetic fields. They need no medium and all travel at the same speed in a vacuum: 3.0 × 108 m s−1. In order from long wavelength (low frequency) to short wavelength (high frequency) they are:

radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays.

A memory aid is "Raging Martians Invade Venus Using X-ray Guns".

Part 2 of 7Common properties

1
They are transverse waves.
2
They transfer energy from a source to an absorber, without needing a medium.
3
They travel at 3.0 × 108 m s−1 in a vacuum (and almost the same in air).
4
They obey v = f λ, so frequency and wavelength are inversely related.
5
They can be reflected, refracted and diffracted.

Photon energy and the danger of the radiation increase with frequency. Gamma rays carry the most energy per photon and radio waves the least.

Part 3 of 7Visible light

Visible light covers wavelengths from about 400 nm (violet) to about 700 nm (red).

Visible light
In order: red, orange, yellow, green, blue, indigo, violet.
Red has the longest wavelength and lowest frequency in the visible band; violet has the shortest wavelength and highest frequency.
When white light passes through a prism, each colour is refracted by a different amount, with violet bent the most,
  • which spreads the light into a spectrum.

Part 4 of 7Producing and detecting EM waves

Different regions are produced and detected in different ways.

1
Radio waves are produced by alternating currents in aerials, and detected by the oscillating currents they induce in a receiving aerial.
2
Infrared is emitted by all warm objects, and detected by skin, thermometers and special cameras.
3
Light is detected by eyes, photographic film and light-dependent resistors.
4
X-rays are produced when fast electrons hit a metal target, and detected by film or digital sensors.
5
Gamma rays come from the nuclei of radioactive atoms, and are detected by a Geiger-Müller tube.

Part 5 of 7Working with the spectrum

Because all EM waves share the same speed in a vacuum, the wave equation lets us convert between frequency and wavelength: λ = c / f.

Working with the spectrum
Radio waves can be many kilometres long, while gamma rays have wavelengths smaller than an atom.
When EM waves move from air into glass they slow down, and the frequency stays the same, so the wavelength shortens.
See Applications of EM Waves and Dangers of EM Waves for what each region is used for and how it harms.

Part 6 of 7Photon energy and frequency

EM radiation can be thought of as a stream of packets of energy called photons.

Photon energy and frequency
The energy of a photon is proportional to the frequency of the radiation.
A photon of gamma radiation has far more energy than a photon of radio waves.
This is why gamma rays and X-rays can ionise atoms while radio waves cannot,
  • and why the dangers of the radiation increase towards the high-frequency end of the spectrum.

The boundaries between regions of the spectrum are not sharp. Wavelengths overlap, and some radiations are named after how they are produced rather than their frequency. For instance, X-rays and gamma rays may overlap in frequency, and the difference is that X-rays are produced by electron interactions outside the nucleus while gamma rays come from changes inside the nucleus.

A useful set of approximate wavelengths

  • radio over 0.1 m;
    • microwaves about 1 mm to 10 cm;
    • infrared about 700 nm to 1 mm;
    • visible 400 to 700 nm;
    • ultraviolet 10 to 400 nm;
    • X-rays 0.01 to 10 nm;
    • gamma rays less than 0.01 nm.

Photon energy and frequency
Learn the order first, then the sizes.
Remember too that a wave changes wavelength but not frequency when it passes from air into glass or water,
  • because the source sets the frequency while the medium sets the speed.
Questions often combine this with the wave equation, so be ready to calculate a frequency from a wavelength using c = 3.0 × 108 m s−1.

Part 7 of 7Worked examples

  1. Calculate the frequency of light of wavelength 600 nm. (c = 3.0 × 108 m s−1)

  2. Radio waves take 1.3 s to travel from the Earth to the Moon. Find the distance.

  3. List the following in order of increasing frequency: X-rays, microwaves, visible light, radio waves.

Key terms

TermMeaning
Electromagnetic spectrumFamily of transverse waves from radio to gamma
Visible lightEM waves detected by the eye, about 400–700 nm
PhotonPacket of electromagnetic energy
Speed of light3.0 × 10⁸ m/s in a vacuum
Nanometre10⁻⁹ m

Avoid these mistakes

1
Saying that red light has the highest frequency in the visible spectrum – red has the lowest.
2
Thinking different EM waves travel at different speeds in a vacuum.
3
Saying that EM waves are longitudinal.
4
Putting X-rays before UV in the order.
5
Thinking that gamma rays have the longest wavelength.

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