Young's double-slit experiment and the discovery of electromagnetic waves

Turning Points in Physics | AQA A-Level Physics

Thomas Young passed monochromatic light through two closely spaced slits and observed an interference patternA pattern of alternating bright and dark fringes produced when two coherent waves overlap. Bright fringes occur where waves arrive in phase; dark fringes where they arrive in antiphase. of bright and dark fringes on a screen.
This could only be explained by the superposition of waves: bright fringes where waves arrive in phase (constructive interference) and dark fringes where waves arrive in antiphase (destructive interference).
Particles cannot cancel each other out in this way, so this experiment provided strong evidence that light behaves as a wave.
Young also used his measurements to calculate the wavelength of visible light, finding it to be around $5 \times 10^{-7}$ m.

James Clerk Maxwell showed mathematically that changing electric and magnetic fields can sustain each other, forming a self-propagating electromagnetic wave.
His equations predicted the speed of such a wave to be:

$$c = \frac{1}{\sqrt{\mu_0 \varepsilon_0}}$$

Substituting the known values of $\mu_0$ (permeability of free space) and $\varepsilon_0$ (permittivity of free space) gave $c \approx 3.0 \times 10^8$ m s$^{-1}$, which matched the measured speed of light.
This was powerful evidence that light is an electromagnetic wave.

Heinrich Hertz confirmed Maxwell's prediction experimentally by generating and detecting radio wavesElectromagnetic waves with wavelengths longer than about 1 mm. Hertz produced them using a spark-gap transmitter and detected them with a loop receiver. using a spark-gap transmitter and a loop receiver.
He demonstrated that radio waves show reflection (bouncing off metal sheets), refraction, polarisation (only transverse waves can be polarised), and interference.
Hertz measured the speed of radio waves by creating standing waves, measuring the distance between nodes (half a wavelength), and using $c = f\lambda$. He found the speed matched $c$.

Fizeau used a rotating toothed wheel to measure the speed of light in air.
Light passed through a gap in the wheel, travelled to a distant mirror and back. If the wheel had rotated so that the next tooth blocked the returning light, the speed could be calculated from the wheel's rotation rate, the number of teeth, and the distance.
The formula is $c = 4dnf$, where $d$ is the distance to the mirror, $n$ is the number of teeth, and $f$ is the rotation frequency at first eclipse.