Why shadow reveals form, and the sun's direction on shadows
Relief cannot be shown without light and shade; how the viewer's line to the sun governs visible shadow
Part Five of the Treatise on Painting continues its account of light and shade. Leonardo finishes a comparison of bodies in dark versus brilliant air, then argues that shadows and lights are the surest means of revealing the shape of any body, since a colour of uniform brightness reads flat and cannot show relief. He closes with a rule of viewing direction: looking with the sun behind you, objects appear free of shadow; looking to the side you see them ringed by light and shade; looking toward the sun's path you see only their shadowed sides.
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Why light and shade reveal the shape of a body
Shadows and lights are the most certain cause for making the shapes of any body known. A colour of equal brightness or darkness cannot display its relief but acts as a flat surface, whose parts, being equally distant from the light that illuminates it, receive equal illumination.
How the viewer's line to the sun governs the shadow he sees
If the sun is in the east and you look west, all illuminated things appear entirely deprived of shadow, because you see what the sun sees. Looking south or north, all bodies appear surrounded by shadow and light, because you see what the sun both sees and does not see. Looking toward the sun's path, all bodies show you their shadowed part, because that part cannot be seen by the sun.