5 decades later, another experiment put Huygens’ model on lead. In 1850, Léon Foucalt compared light speed through air to light speed through water and found that, contrary to Newton’s assertions, light wasn't faster in denser medium. Instead, just like waves, it slowed down. 11 years later, James Clerk Maxwell published On Physical Lines of Force, where he predicted electromagnetic waves' existence. Hhe noted their similarity to lightwaves, leading him to conclude that the 2 were one and the same. It appears that Huygens’ wave model was right, but (1900) Max Planck had an idea that some radiation behaviors by depicting electromagnetic waves energies divided into individual packets.
[5 ](1905) Albert Einstein built on Planck’s concept of energy packets and finally settled the corpuscule-versus-wave debate—by declaring it a tie. Einstein explains light act as both a particle and wave, with energy per light particle corresponding to the wave's frequency. His proof is from the photoelectric effect's studies —how light knocks electrons loose from metal. If light traveled only in a continuous wave, then shining a light on metal for long enough would always dislodge an electron, because the energy the light transferred to the electron would accumulate over time.
But the photoelectric effect didn’t work that way. In 1902 Philipp Lenard had observed that only light above a certain energy—or lightwaves above a certain frequency—could pry an electron loose from the metal. And it seemed to do so on contact, immediately. Here, light was acting more like a particle, an individual energy packet.
Convinced by the light wave model, Robert Millikan set out to disprove Einstein’s hypothesis. Millikan took careful measurements of the relationship between the light and electrons involved in the photoelectric effect. To his surprise, he confirmed each of Einstein’s predictions.
Einstein’s study of the photoelectric effect earned him his sole Nobel Prize in 1921.
In 1923, Arthur Compton provided additional support for Einstein’s model of light. Compton aimed high-energy light at materials, and he successfully predicted the angles at which electrons released by the collisions would scatter. He did it by presuming the light would act like tiny billiard balls.
Chemist Gilbert Lewis came up with a name for these billiard balls. In a 1926 letter to the journal Nature, he called them “photons.”
The way that scientists think about photons has continued to evolve in more recent years. For one, the photon is now known as a “gauge boson.”
Gauge bosons are force-carrying particles that enable matter particles to interact via the fundamental forces. Atoms, for example, stick together because the positively charged protons in their nuclei exchange photons with the negatively charged electrons that orbit them—an interaction via the electromagnetic force.
Secondly, the photon is now thought of as a particle, a wave, and an excitation—kind of like a wave—in a quantum field.
A quantum field, such as the electromagnetic field, is a kind of energy and potential spread throughout space. Physicists think of every particle as an excitation of a quantum field.
“I like to think of a quantum field as a calm pond surface where you don’t see anything,” Ruiz says. “Then you put a pebble on the surface, and the water pops up a bit. That’s a particle.”
