Electromagnetic Waves

Physics: AQA A Level, topic 12.2.2

Fizeau and Maxwell

Fizeau and Maxwell helped to discover the speed of light.

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.

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

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

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.

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.