Shade illusion

Leading edge of the shade

When you look at the light-shade boundary between the shaded and the lit areas, there seems to be a narrow band of shade on the shadow side of the boundary that is darker than the rest of the shaded area.   Likewise, there seems to be a narrow band of light along the lit side of the boundary that is brighter than the lit area.

The light-shade boundary itself is not sharply defined.