Sketch, please explain why a person's head side is illuminated to produce such a light and shadow effect on his face. !

The area you marked is the inclined side of the face, half bright and half dark, belonging to the gray surface. Reading anatomy books or sliced plaster heads can help you understand. Light and shadow are produced because of structural changes, so understanding the structure is the basis of describing light and shadow. There is no point in simply discussing light and shadow. If you don't understand the structure, you can't draw it by changing the position of the light source. So we should grasp the essence-structure and form.