Poisson Spot Intuition
Why a bright spot appears in the center of a circular shadow.
Observation
When light passes around a circular object, a bright point can appear at the center of the shadow. This is called the Poisson spot.
Intuition
Light diffracts around the edge, and waves from different sides can constructively interfere at the center line behind the object.
My practical takeaway
Counterintuitive effects often reveal hidden assumptions. If a result looks impossible, check the model assumptions before rejecting the result.