Look at the picture below. It shows one of two gimballed oil lamps we have on board, and the light patterns on the wall when the sun streams through the portholes and the lamp's globe. We see the same patterns with both lamps and with light coming from any one of six portholes.
As you can see, the pattern is dominated by horizontal lines. No such lines are visible in the glass. Both oil lamps make similar patterns. I've tried cleaning the globes with vinegar to see if it might be dirt on the glass causing the lines. Nope.
The porthole glass is tempered. I suppose it is possible that the light coming through them is polarized. But I can't see how that plays any role. Besides, the different portholes would have random polarization orientations, but the lines are horizontal no matter which porthole the light comes in.
You can see in the picture that light from the portholes that does not go through the lamp globe has no pattern.
I just noticed that the picture shows something that is not visible to my naked eye. I see spider-web like connections between the lamp and the wall. It looks like the light rays cross forming an X pattern in the air. Hmm, very interesting but I still don't see how that makes the lines.
Could it be constructive/destructive interference as light rays reach the wall via two different paths?
To me, this is an unexplained mystery. Why the lines in the light pattern? Is this something a PHD candidate could write a thesis on? I could spend days staring at that pattern trying to understand it. What do readers say? Please comment.