On a clear morning, light comes through this window (viewed from the inside)
It then hits the door on the opposite side (so viewing from the outside the door straight on the other side)
There is nothing particularly special going on.
However, if I go through the door and turn around to face the door, I see this
Light seems to be dispersing into the color spectrum solely because of the hole in the wooden door. Some days it's even clearer than it is today.
What gives? I know light can do this through prisms, but how come it's happening with only a hole in a door?
As per Floris' instruction, I did a little more experimenting. Here are the results (extra hint/spoiler):
It's far from perfect because I made the hole using a black gym bag and some masking tape, and used a handheld mirror to reflect it onto the door, but I think it shows his explanation to be correct pretty well!