Imagine shining a light through a single slit cut out of a board in order to see the pattern of light it creates on the solid wall behind it. What results is something you might call the shadow puppet effect. Not unexpectedly, a single sliver of light matching the shape of the slit appears on the wall. Simple enough.

Yet when a second, identical slit is cut parallel to the first, the expected result—two slivers of light—is not what we actually get. Instead, what appears on the wall is an interference pattern—multiple (more than two) slivers of light of varying intensity. This is the double-slit experiment. [Read more…]