The room in the basement I work in during the summer is lit by a bare light bulb on the ceiling. I noticed during a Zoom call that it looked like it projects a vertical beam straight down. But bulbs obviously project in all directions, so that didn't quite make sense.
I made a guess that sunlight coming in from the basement window crosses the light it projects downward and that intersection is picked up by the laptop camera. But that also seemed dubious.
So, I turned off the bulb and saw three vertical beams stemming from the window in the other room behind me. Was that the actual source of the beam?
No. When I turned the bulb back on, there was a single beam, clearly intersecting the bulb.
So the bulb is clearly the source but the mystery of why the camera picks up a vertical beam remains!