Light and shadow are two of the most challenging problems facing a painter. Painters can’t harness real light and shadow; instead they must rely on subtle gradations of color to create the illusion.
Rembrandt, Self-Portrait, 1634. Galleria degli Uffizi
In general, I use “light” in painting to mean all those areas which are directly lit by a light source. For example, in this Rembrandt self-portrait most of his face, the glow behind him, some of his hair, and the front of his coat are in the light. The “shadows” are all those areas which fall outside of the light. To demonstrate the division between light and shadow, I cut and pasted squares of color taken from this painting, and divided them into two groups below.