Light is a type of energy, which makes the sense of visibility.

When the light rays incident on an object, some part of it reflects from the object.When these reflected rays approach to observers eyes, the image of object forms.

Whereas, shadow of an object forms when light does not reach at any part of the object.

So, no light from that part enters in observers eyes, and forms shadow.

So, we can say, Shadow is a dark patch formed on a surface behind an opaque object placed in the path of light.