Light can strike an object from many different directions, but it is the strongest light-the primary light source-that best defines an object. Thus, to firmly establish form in a drawing, emphasize the primary light source, then hint at other light sources.
The most common and, when you are depicting form, useful of the other light sources is the light reflected back to an object from nearby surfaces. Light from the primary source that does not fall on an object continues past it into the environment, and some of this light is bounced back as reflected light. Most convex objects will have some reflected light within their shadowed side. The lighter the area reflecting the light, the greater the amount of reflected light on the object.
It would seem logical to assume that the part of an object farthest from the light source would be the darkest. But this is not the case. Because of reflected light, the darkest dark of a convex object is usually within the object, not on its edge.
It would also seem logical that the lightest part of an object would be on the side nearest the light, but this is not so. The light striking the side of an object nearest the light source is bounced back toward the light source. The light on the lightest part we see must bounce off a surface somewhere between the angle of the light source and the angle of our vision. Because of the path light must follow to get to our eye, the brightest part of a lit object is not its edge.
We have said that the lightest light and the darkest dark rarely appear on the edge of an object; it follows, then, that on a convex object, the value contrast is stronger on the middle of the form than at its edges Thus, the middle of the form appears to come forward, while its edges appear to recede. As contrast between light and shadow is increased, so is the illusion of three dimensions.
Reflected light comes in the same shapes on the basic forms as other lights and shadows; on a cone, for example, it appears as a triangle.
Reflected light bounces back from the environment and lights the shadow side of objects. Reflected light is never as strong as the primary light.
The lighter the reflecting surface, the greater the amount of light it will reflect.