Electromagnetic Waves

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Fizeau and Maxwell

Fizeau and Maxwell helped to discover the speed of light.

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Fizeau’s experiment

  • Fizeau set up an experiment using a rotating cog and a beam of light to measure the speed of light.
  • He shone a beam of light through the gap between two teeth of the cog.
  • This beam reflected back from a mirror, and back through a gap in the cog.
  • He adjusted the speed of the cog until the reflected light was blocked completely by a tooth.
  • He could then use the rotational frequency and the distance that the light had travelled to calculate the speed of light.
Illustrative background for Maxwell's theoryIllustrative background for Maxwell's theory ?? "content

Maxwell's theory

  • Maxwell predicted that electromagnetic waves existed, and also predicted their speed.
  • He believed that the speed of electromagnetic waves in a vacuum was:
    • c = 1μ0ϵ0\frac{1}{\sqrt{\mu_0 \epsilon_0}}
    • c = 3 × 108 ms-1
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Fizeau and Maxwell

  • Fizeau’s observed speed of light was very close to Maxwell’s estimation.
  • This was evidence that light was an electromagnetic wave.

Hertz's Discovery of Radio Waves

Hertz not only discovered radio waves, but also measured how fast they travel.

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Hertz's discovery

  • In 1887, Hertz discovered the radio wave.
  • Using an induction coil, he showed that sparks could cross a gap of air.
  • He found that these waves could induce a potential difference, and so had a magnetic component.
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Measuring the speed of radio waves

  • Hertz set up radio wave detectors at regular intervals, and managed to construct a stationary radio wave.
  • He then measured the wavelength, and by using a fixed resonant frequency he could measure the speed using:
    • v = fλ
  • The speed lined up with Maxwell’s prediction, and provided evidence that radio waves were another type of electromagnetic wave.

Jump to other topics

1Measurements & Errors

2Particles & Radiation


4Mechanics & Materials


6Further Mechanics & Thermal Physics (A2 only)

7Fields & Their Consequences (A2 only)

8Nuclear Physics (A2 only)

9Option: Astrophysics (A2 only)

10Option: Medical Physics (A2 only)

11Option: Engineering Physics (A2 only)

12Option: Turning Points in Physics (A2 only)

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