Quick: is light a particle (like a speck of dust) or a wave (like a radio transmission)?

The most honest answer is: yes. So fickle. Light sometimes acts like a particle and sometimes like a wave—but science didn't decide this for a long time.

In the late seventeenth and early eighteenth century, two main theories were proposed. You may remember a man named Isaac Newton—powdered wigs, apple, couple of things named after him. He was in one corner of the debate, arguing that light was a particle (largely set forward in his book Opticks and its sequel Isaac Newton and the Goblet of Fyre). Newton said that light beams were made up of a line of very, very fast particles, think full auto BB gun, that he called "corpuscles." This explained the way light reflected: bouncing off the mirror surface like a billiards ball off a pool table wall. It also explained how light refracts in something like water: bending away from its original path and distorting the images you see. It even explained a prism splitting white light into a rainbow of different colors—different color particles must have different masses, said Newton. ¹

But Christiaan Huygens, the Hague horologist, wasn't having any of Newton's corpuscular nonsense. He thought light was a wave and said so in his own book, Traité de la lumère and its sequence Huygens en de Gevangene van Azkaban. The wave description explained things that Newton glossed over or didn't address, such as how the interference patterns that light makes are almost identical to those made by ripples on a pond.

Newton won a lot of points in the scientific community for his invention of this thing called gravity, though, and so it was his idea—light as a stream of particles—that dominated scientific discourse for decades after his book was published. But in the early 1800s, a physicist named Thomas Young performed an experiment where he shined a light through two narrow slits in a plate. The pattern on the other side of the plate was identical to what pond ripples would create—just as Huygens predicted. ²

Science then decided that, gravity or not, Newton probably messed up on his idea of light-as-particle. That is, until a guy named Albert Einstein came along and described how light hitting metal can eject electrons from the metal, something called the photoelectric effect. This ended up better explained by light being a particle. ³.

As it turns out, light (and even things classically thought of as particles, like electrons) have both wave and particle properties. We call this wave-particle duality, and it took a lot of very smart people a very long time to figure it out—because, ultimately, almost everyone was right.