When a
wave hits an object
then the region immediately behind that object is protected
from the wave and doesn't experience it at all. Shadows
are an example of this. Waves of light are blocked by
something physical, so the region immediately behind
the object doesn't experience those waves and is darker.
But shadows are sharper close to an object
than they are further from it. This is due to diffraction.
Waves that pass the object change their direction of
travel slightly. The wave that just missed the object
spreads in a circle or sphere, into the space behind
the object. This is why shadows become more blurred
further away from the object that casts them. Eventually
the spherically spreading waves from each edge of the
obstacle may even meet up.
