Darkness is absence of light. Shadow is diminution of light.
Primitive shadow is that which is inseparable from a body not in the
light. Derived shadow is that which is disengaged from a body in
shadow and pervades the air. A cast transparent shadow is that which
is surrounded by an illuminated surface. A simple shadow is one
which receives no light from the luminous body which causes it. A
simple shadow begins within the line which starts from the edge of
the luminous body _a b_.