Eli5: What does it mean when light is “observed”? what counts as an observer?

Question

Eli5: What does it mean when light is “observed”? what counts as an observer?

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Answer 1

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.