How can this circle light cast a triangle shadow? A friend of mine sat in the lobby of a local hospital and noticed that the circular ceiling lamp cast a triangle-shaped field of light / shadow (depending on how you look at it I guess).
How is this possible?
 
 A: While the bottom of the fixture (chandelier?) has circular symmetry, the top of it has this symmetry broken.  Note that there are three support connections between the fixture and the ceiling, and that the edges of the bright triangle are roughly parallel with the lines between the three ceiling supports.  Apparently something on the hidden topside of this fixture has triangular symmetry.
Also note that the support cables/rods do not cast obvious shadows, even right at the places where they attach to the ceiling.  That means that the bright spot is not from a single source, but from very many small sources --- probably arranged in the ring and aimed at the center spot on the ceiling.  But that doesn't address the question of how the shadows are made.
The light fixture has clearly been designed to create this pattern because it is improbable and cool-looking.  A photo from above would answer the question right away.  So would (probably) finding the fixture for sale in some catalogue.  Reinventing the design in the answers here might or might not come up with the same solution as the artist who came up with this clever artistic object.
A: Welcome to StackExchange! The best explanation is that the Illuminati run this hospital and so they have used arcane magic to defy physics...
In all seriousness, it's hard to be sure with the only perspective that you give. My best guess would be that there is a diffuse light source with a triangular aperture below the circular light on the ceiling. Since the source is diffuse, and not a point, the shadow from the circular light could be softened to the point of being hardly perceptible. This is exactly why photographers use those lamps with fabric around them to make a big white square.
A: There's another light above the t.v. screen. The triangle may be a reflection of that light off the top left corner of the screen.
