This morning, my partner and I noticed a perfectly shaped semi circle of light on our bedroom wall. As the sun rose this quickly became a full circle. I could not see what was causing this shape so traced it with my hand back to a small vertical slit in out curtains. With a piece of paper I confirmed that the shape of the light patch was a slit right up by the curtains, but as I moved across the room it gradually morphed into a circle!
I thought it might be something to do with single slit diffraction, but I cannot find any exapmles that seems to create such a perfect circle and has no other visible artifacts.
Any ideas?
