If we move the light source such that it is coming from higher up and over to the right (positive x and positive y), we immediately notice that the polygon is darker on the surface. The shadow cast by the normal (here, the z axis) helps us to understand the change in the direction of the light. Because the direction of the light is at an angle to the normal, light intensity on any spot on the polygon is reduced.

Taking the light source even higher so that it is almost edge-on to the polygon, the surface now falls into deep shade and the shadow cast by the z axis lengthens. This process is exactly what happens when the sun sets over the horizon.

Let's replace our single surface with a cube of six surfaces. Notice how the x, y and z axes all serve as normals to the six surfaces in this picture. Once again there is only one directional light source. Examine this picture carefully. A 3-D artist quickly learns to read the lighting of a scene, imagining the placement and direction of lights from viewing the rendered image. Note how sophisticated the lighting can feel with only a single directional light and a little ambient light. Ambient light is non-directional light. It is light that flows into all corners and casts no shadows. Rooms are typically filled with ambient light, the result of light sources reflecting off of walls and furnishings.

3-D effect depends enormously on lighting. We instinctively sense depth by reading shadows, and we derive great esthetic pleasure from gradients in lighting. A slowly animated version of this cube reveals the full play of shadow and the effect of the change of angle between the surface normals and the direction of the light. The axes turn along with the cube so that they remain normal to its surfaces.