The Staircase
Nine bars of grey, each one step lighter than the last.
How to play
Look first, then answer
Nine bars of grey, each one step lighter than the last.
- Under a minute · one question · then the explanation
How it works — best read after you have tried it
Edges, sharpened
Every bar is a single flat grey. Where two bars meet, the lighter one looks lighter still along the edge and the darker one darker still — thin bands that are not in the picture.
The physicist Ernst Mach described them in 1865. Here the old explanation holds up: cells in the retina are held back by their neighbours, which exaggerates any step in brightness, and so the visual system sharpens edges before anything else sees them.