In another puzzle, you had to use either balls or water to determine the amount of slits in a certain box. It should have been visible that the interaction with the slits works differently for water waves when compared to normal balls. The normal scattering with balls is more or less familiar and behaves as might be expected from daily life. Waves on the other hand are a lot less familiar and sometimes behave in counterintuitive ways. Even so, they can still somewhat be understood. The reason both were used is because the smallest particles, like photons or electrons, behave like both depending on the situation. As explained previously for the diffraction grid, when light hits these slits, it normally behaves like a wave. Therefore, it was believed for a long time that light was a wave. Initially, there was actually some debate about whether light is a particle or a wave, with prominent scientists, like Isaac Newton believing it to be a particle, while Christiaan Huygens was a proponent of the wave theory. When double slit interference was studied in more detail by Thomas Young, the wave nature seemed to be confirmed, particularly as the propagation of light as described by Maxwell’s equations fulfils the wave equation. Eventually, another experiment once again cast doubt on these findings. There were multiple discoveries that once again suggested that light is discrete (aka a particle). The most important were black body radiation discovered by Max Planck and the photoelectric effect discovered by Albert Einstein. In both photons seemed to have discrete effects. Ultimately, even the double slit didn’t seem quite as decisive anymore. It was found that when one places a detector at each slit to measure where a photon travels through, the pattern all of a sudden resembles a pattern that might be expected from particles, like balls, that move with a wide distribution of angles.
(source: Feynman lectures)
Because of these discoveries, the debate was reignited. Only in the 1920s were the two models unified by quantum mechanics. Since then particle-wave duality is the best model. Originally proposed by Louis de Broglie, it was suggested that photons can be seen as wave packets that can describe both wave and particle behaviour under certain circumstances. Schrödinger’s equations later provided a mathematical framework to help describe this phenomenon in more detail.