The Impressionists were concerned with capturing the qualities of light and color in their paintings. Since the colors change so much depending on whether the object is in direct light or in shadow, one of our first challenges was to make several black and white studies, starting with a rectangular block, and delineating the difference between the lit side and the shadow side.
It sounded like an easy exercise, but I found it harder than I expected. The light side includes all of the area lit directly by the light source. Half-tones, formed when the plane of an object turns away from the light source, are still considered part of the light. The shadow side includes reflected light bouncing in from other objects. Especially in a round object, the light bouncing back off the tablecloth into the shadow made it difficult to determine where the shadow started.