Eli5: What does it mean when light is “observed”? what counts as an observer?
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As I understand it, on a very basic level being observed in this context just means that it interacts with something else–runs into another particle, something like that. My understanding is that it more specifically means that it has an interaction that requires it to adopt specific properties (“the wave function collapses”) instead of vague, quantum ones (the wave function). It’s possible to have interactions that don’t require it to have specific properties in which case the light and the thing it’s interacting with become part of the same quantum system, which itself will collapse as soon as any part of it becomes observed.
All of which sounds bizarre and very unlike how we understand the Universe to work generally. Not to mention the weirdness of interactions that collapse the wave function vs. interactions that are just kind of cool with everything being vague and smudgy. But that’s quantum physics for you: strange and deeply unintuitive.
It means detected. You use an instrument to detect light. One example is a single photon counting module where a single photon is absorbed and converted to a large electrical signal. By doing so, you know roughly where and when you detected it.
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