eli5: Why is it significant that light is both a wave and a particle?
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Because it is counterintuitive, and was a debate for a long time, back in the day when people thought something was either described as a wave or a “solid” particle, before being accepted as true. A “pure” particle (theoretical, because they don’t exist) couldn’t self-interfere in a double slit experiment, while “pure” waves didn’t do things like point interactions (like being absorbed by another particle).
We’ve since found out that nothing is a pure particle, and pretty much all waves can have “quasi-particles” (as in, they don’t have all properties a “real” particle have, just some) assigned. And while that might seem weird, the “particles of sound” phonons actually have real quantum effects in supercondutors. It’s a topic waaaay beyond ELI5, but I’m mentioning it to show it’s not just a fun theoretical idea, it has real effects.
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