Today's Inktober prompt is "camera". I expanded it a bit to "camera obscura/chambre noir".
One day, a long time ago, I entered a dark windowless corridor in the new, but already shabby, building of our university. One of the doors had a hole, where the lock was broken out completely. The room behind the door had the windows looking at another part of the same building, the sun hitting it full-on. First, I noticed a blurry regular pattern on a wall opposite to the door and then something moving. It took me some time to work out the moving thing was a curtain hanging out of one of the windows in the sunlit wall. The fact that it was upside down didn't help to recognise it.
It was an accidentally formed camera obscura, very imperfect of course. I'd never seen the effect before. We had studied a bit of optics at school, and there might have been an illustration in the textbook, but it probably was a line drawing, and it was difficult to form a link between it and real life. My own illustration of the event also doesn't explain much, I'm afraid.
And below I leave a beautifully executed, but still not very explanatory, version of the same phenomenon printed by Hokusai