12.2.2

# Electromagnetic Waves

Test yourself

## Fizeau and Maxwell

Fizeau and Maxwell helped to discover the speed of light.

### 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.

### 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 = $\frac{1}{\sqrt{\mu_0 \epsilon_0}}$
• c = 3 × 108 ms-1

### 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.

### 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.

### 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.