So until around 1902, it was near unanimously agreed that light was a wave, because it does all the stuff that waves do, like diffracting — we wouldn’t have rainbows, or the cool Pink Floyd album cover with a prism splitting light into a rainbow otherwise.

What changed in 1902 is that an experiment (called the photoelectric effect, if you’re curious) produced results that would have only been possible if light was a particle. The photoelectric effect had been observed a bunch of times through the 1800s, but in 1902, a variant of the experiment produced results that would be impossible to explain if light were a wave. So then people start asking “okay, maybe we were wrong, maybe light is actually a particle”. Except that didn’t square with the centuries of evidence showing that light was a wave.

It turns out that light is both a particle and as wave. Or maybe neither. Because the key concept here is that particles and waves don’t exist. They’re just conceptual categories that we made to put boxes around phenomena to make them more understandable, much the same way that binary gender is a simplifying framework that works until it doesn’t.

Now, this doesn’t mean that the underlying phenomena, like light being diffracted, or the photoelectric effect, aren’t real. The problem was in our framework of how we labelled them. Once physicists got their head around the possibility that light could be both a particle and a wave, they realised that there were a bunch of other situations where we could model light as a particle and discover interesting stuff. Most people don’t need to understand this, because the simplified model of everything being either a particle or a wave works well enough that even if it’s not correct, it’s still useful — these categories developed for a reason, after all. By analogy, it’s like if I said “women have breasts”. It’s true in most instances, so it can still be a useful observation, even if it’s not strictly accurate.

However, it gets even more interesting. At first, scientists thought that light must just be a special kind of phenomenon, able to exhibit both particle and wave characteristics. But then, in the double slit experiment, they found that under certain circumstances, electrons (which were near unanimously considered to be particles) could diffract — i.e. act like waves. This was the result that really drove home the notion that when we’re studying stuff that are super small and specific, our existing rules and categories sort of fall apart. It’s even been suggested that other things that we squarely consider to be particles could show wave nature too, but the larger you get in scale, the harder it is to observe quantum phenomena (which basically just means that our rules work well when they’re applied to the circumstances we developed those rules under. “Quantum phenomena” mostly just means “shit that happens when we’re so zoomed in that our existing frameworks stop working”)

In a sense, we could say that light behaving as a particle is analogous to a non binary man, and electrons behaving as a wave is analogous to a non binary woman. Maybe it would be more sensible to dispense with these categories entirely, but there are many phenomena and many people who find the terms useful.