AQA GCSE Physics · Electromagnetic waves

How radio waves are made and received

An alternating current in a transmitter makes electrons oscillate. The oscillating charges send out a radio wave. When the wave reaches a receiver aerial, it makes electrons there oscillate at the same frequency.

Physics 8463: 4.6.2.1–4.6.2.3 Combined Trilogy 8464: 6.6.2.1–6.6.2.3 Production & absorption: Higher tier
Electrons in the aerial Radio wave (oscillating field) Animation slowed down about 100 million times
Frequency f
100 MHz
Wavelength λ
3.00 m

Speed of radio waves in a vacuum: 3.0 × 10⁸ m/s. The speed stays the same as you change the frequency, so the wavelength has to change.

Oscilloscope

Transmitter current Receiver current

Both currents alternate at the same frequency. The receiver current is smaller, because the wave's energy spreads out, and it lags behind by the time the wave takes to travel.

What the specification expects

Key points to learn

4.6.2.1 Properties of EM waves 1

Radio waves are EM waves

  • Electromagnetic waves are transverse.
  • They transfer energy from the source to an absorber.
  • All EM waves travel at the same speed in a vacuum or air: 3.0 × 10⁸ m/s.
  • Radio waves have the longest wavelength and lowest frequency in the EM spectrum.
4.6.2.2 Properties of EM waves 2 · HT only

Production and absorption

  • Radio waves can be produced by oscillations in electrical circuits.
  • When radio waves are absorbed, they may create an alternating current with the same frequency as the radio wave.
  • So radio waves can induce oscillations in an electrical circuit.
4.6.2.3 Uses and applications

Why we use them for radio and TV

  • Long wavelength, so they pass through walls and around buildings.
  • Not absorbed much by the body, so they are safe at broadcast strengths.
  • Some wavelengths reflect off the ionosphere, so they can travel very long distances.
Calculation practice

The wave equation

v = f λwave speed (m/s) = frequency (Hz) × wavelength (m)

For radio waves v is always 3.0 × 10⁸ m/s, so rearrange to find the wavelength:

λ = v ÷ f

Convert to hertz first. 1 kHz = 10³ Hz, 1 MHz = 10⁶ Hz, 1 GHz = 10⁹ Hz. A higher frequency always means a shorter wavelength.

Wavelength calculator

Try:
Explaining suitability

How different radio wavelengths reach you

Useful for 6-mark “explain why” answers. Different wavelengths behave differently, so each band suits a different job.

IONOSPHERE (charged layer of the upper atmosphere) Transmitter VHF / UHF: FM, DAB, TV line of sight to a nearby mast Short wave: sky wave reflects off the ionosphere, reaches other continents Long wave: ground wave diffracts around the curve of the Earth
Misconceptions

Common mistakes

“Radio waves are sound waves.” Radio waves are electromagnetic.

Sound is longitudinal and needs a medium. A radio turns the electrical signal from the aerial back into sound using a loudspeaker.

“A higher frequency radio wave travels faster.” Every EM wave travels at 3.0 × 10⁸ m/s in a vacuum.

Raising the frequency shortens the wavelength. The speed does not change.

“The receiver makes its own current.” The absorbed wave induces the current.

Energy carried by the wave makes electrons in the receiver aerial oscillate, giving an AC of the same frequency as the wave.

“A direct current in the aerial makes radio waves.” It has to be an oscillating (alternating) current.

Only oscillating charges emit a wave. A steady current produces no radio wave.

Check understanding

Quick quiz

Score: 0 / 6
Exam practice

Exam-style questions

Higher tier

Describe how a radio transmitter produces radio waves, and how a radio receiver detects them.

[4 marks]
Show mark scheme
  1. An alternating current / oscillating electrons in the transmitter circuit (aerial) (1)
  2. produces radio waves with the same frequency as the oscillations (1)
  3. radio waves are absorbed by the receiver aerial (1)
  4. inducing an alternating current with the same frequency as the radio wave (1)
Foundation and Higher

A local radio station transmits at a frequency of 1.2 MHz. Radio waves travel at 3.0 × 10⁸ m/s. Calculate the wavelength of the radio waves.

[3 marks]
Show mark scheme
  1. 1.2 MHz = 1 200 000 Hz (1.2 × 10⁶ Hz) (1)
  2. λ = 3.0 × 10⁸ ÷ 1.2 × 10⁶ (1)
  3. λ = 250 m (1)

An answer of 250 m scores all 3 marks. An answer of 2.5 × 10⁸ m means MHz was not converted.