A shadow is simply the region behind an object
where light is not due to blocking by an opaque
object. It might be easier to think of light
as photons, and the shadow as the space which
photons from the light source cannot reach
due to the occluding object. Although a shadow is
often thought of as a 2D area, that is only a
reverse projection onto a surface; an object's
shadow is actually the entire volume where light
is blocked. There are different types of
shadows though, called the
umbra and penumbra. The former is the
region which has no light at all, and the
latter is the region which can be reached by
some light, but less than an unobstructed
The sizes of these regions depends on the size
of the light source and the distance from the
A shadow requires a source of light and an
object. The shadow is formed by the object
blocking the light, and the shadow will
always be in an area farthest away from the light,
and of course, the shadow will often be darker
than the area around it and marked by a boundary.
If there are more than one source of light,
there will be more than one shadow. If there is
light from every direction, like surgical lamps
are designed to give, there would be no shadow.
If there is no light, there is no shadow,
strictly speaking. Shadows can be multicolored or
three-dimensional as well, and those are
fascinating from a both physical science
perspective and an artistic perspective.
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