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This morning I noticed strips of light forming on a piece of paper. The only source of light was one window, with 1/3 of it open. So light was coming from either an insect screen or blurred glass. Also no direct sunlight because of tall buildings around.

When I lifted the paper so that the surface was orthogonal to the source of light, the strips became more dense.

The strips only appeared at a narrow range of distance (around $3\;\text{m}$ or $10\;\text{ft}$) from the window.

Only knowing high school physics, at first I thought it was light interference, but then the insect net is a grid, not slits! What is happening?

Picture 1: more or less evenly spaced light strips with a vertical column of shadow in the center, which is probably from a security rod in my window (Picture 2)

strips of light appearing on paper, facing light source

window at the time of the event. I live in Seoul.

user30987
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    Can you take a picture of the window as it was when you saw the pattern? – Mark H May 19 '18 at 03:47
  • Is it a metal screen? I have just finished using some metal insect net, and although it is a grid , depending on way I hold it , one line direction is shiny,although intermitent , due to the grid. At what distance from the scree is the photo? – anna v May 19 '18 at 05:23
  • @MarkH I added a picture of the window. Thank you for showing interest! – user30987 May 19 '18 at 09:12
  • @annav The distance was around 3m(~10ft) and the screen is indeed metal, though extremely dusty. Thank you for showing interest! – user30987 May 19 '18 at 09:13
  • A couple of close-up photos of the net from different angles might help. The detailed shape of the holes and the wires could be the cause. – S. McGrew May 19 '18 at 13:58

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