This practice made it possible for artists to create an illusion of depth by simply following a standard set of rules for applying light and shadow to a form.
Highlights are easiest to see on reflective or glossy surfaces.
We see more of the light side than we do shadow in the illustration at the top right. This is the center of the darkened area. From our vantage point it is a thin crescent. It can help to define the back edge of an object. It occurs in a shadowed area and is caused by light being reflected from another area or created by a secondary light source.