
When I teach color theory, I always explain to the students, that in art there is an important difference between shade and shadow.
When light falls on an object, it illuminates the side that is facing the light. The other sides of the object, which are not pointing towards the source of this light, are in
shade.
The object will, in turn, cast a
shadow onto the surface which is underneath or behind it.
The change between an area in light and an area in shade can be soft and gradual if the object is rounded (like the forehead), or abrupt if it has corners or pronounced plane changes (like the violin).
Cast shadows have sharp edges close to the object that casts the shadow (like the cast shadow of the bow on the violinist´s arm) and softer edges the farther away from the light source the shadow gets (like in the case of the cast shadow of the arm and violin on the red curtain).
The handling of the brush strokes, using soft and hard edges, has to be according to that principle in order to give a realistic impression.