Some 100 years after the time of Newton, French physicist Augustin-Jean Fresnel (1788 to 1827) asserted that light waves have an extremely short wavelength and mathematically proved light interference. In 1815, he devised physical laws for light reflection and refraction, as well. He also hypothesized that space is filled with a medium known as ether because waves need something that can transmit them.

In 1817, English physicist Thomas Young (1773 to 1829) calculated light's wavelength from an interference pattern, thereby not only figuring out that the wavelength is 1 micron or less, but also having a handle on the truth that light is a transverse wave.

At that point, the particle theory of light fell out of favor and was replaced by the wave theory.