Light in painting is created by using white paints, and/or colors paled with white paint. The paint can be applied near or on or beneath the apple or the child's face or the violin or the open book. An artist reproduces with intention sunlight, or moonlight, or candlelight, or electric light and such.
In the experience of painting I've noticed that occasionally, light picks its own home; light gathers somewhat cheerfully in blank spots of background. There is contrast between the paint and paper that is predictable. And sometimes in the breaks of a brush stroke, or in the patch of emptiness between two painted areas, a powdery paleness will glimmer, and take on a spirited life.