Most objects in the world do not let much light pass through so that these objects cast shadows. The exact shape and description of the shadows changes depending on the direction of the light, e.g. when outside shadows are long and go to the west in early morning, short and northerly at noon, and then long but to the east in the late afternoon. However, there are certain general rules about shadows. First, in a place with only one source of light, e.g. outside, the shadows from all the objects in the area all go in the same direction. As a corrollary of this rule, it is certainly true for all natural lights, and for most artificial lights, that the light comes from above to some degree. We do not usually experience getting illuminated from the floor. Second, for a solid object sticking up the side of the object in shadow is the side away from the light but for a hole in the ground, the shadow is on the side near the light. The figure to the right of this paragraph attempts to illustrate this idea, but you can better see it with a coffee cup which is both standing up on the outside and has a hole inside where you pour the coffee. Notice the shadows. On the outside, the shadows are deepest on the opposite side from the inside of the cup.

The animation below illustrates how our assumptions about the direction of light plays a role in how we perceive an object. This image is of a stone carving found in many places in the United Kingdom.  These rosettes are a variation to the more common cup-and-ring motifs which were pecked out in the outcrop rock in the late Stone- and early Bronze age (3500-2000BC) with stone tools. This one is from Ormaig, north west of the village of Kilmartin in Argyll on the west coast of Scotland. When the picture was taken, the light came from the left as indicated by relative position of the sun drawn below. The figure is made by carving into the stone so the image is a series of indentations. When the image flips the viewer still assumes that the sun is from the same direction. However the shadows are consistent with bumps not an indentations and so the figure looks like raised bumps not to be carved into the stone. Let the image flip a few times and see for yourself.

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Photographs by Gus van Veen & Jan Brouwer, Used by Permission, "Rock Art in the British Landscape"

Shadow can play a very powerful role in defining form by giving the object a three-dimensional feel as in the title to this page. In addition, artists can take good advantage of shadow to define form by highlighting how different portions of an object are at different depths and therefore the object closer to the light will cast a shadow on the more distant object. The etching below by M.C. Escher of a leaf uses shadow as the primary means of defining the form of the leaf that is in the sun. If you look at the portion of the leaf that is lit up you will notice that lines are not drawn for the veins in the leaf but shadow defines where they are.

Shadow can play a broader role in defining depth between objects since objects that are in shadow must be farther from the light than objects that are not in shadow.

Dewdrop (Dauwdroppel) by M. C. Escher