As I wrote before, We see objects only because they all reflect light.
After beam of light starts its way from a light source, it touches objects, reflects and reaches our eye.
sometimes these reflected beams of light cannot reach our eye, because of other objects between the object and our eye. and we don't see a part of that object.
what exactly happens when the light cannot reach our eyes?
Imagine that we are watching at a wall, in a dark room, with one lamp...
light which comes from a light source, a lamp, reaches the wall, reflects from it, and reaches our eye.
1. we put our hand between the wall and our eye, we will not be able to see a part of the wall, because "the reflected" light cannot reach our eye. But we will see our hand, because a light, reflected from, our hand reachs our eye.
2. we put our hand between the wall and a lamp, the light from the lamp, will not reach the wall, and instead we will see a dark spot. why? because there is an obstacle, our hand. that's why that spot will have a shape of our hand.
Border between a light and a shadow is YOU! (Stanislav Yezhi Lec, "Good and Evil")
Aziz Shahhuseynov. special thanks to Franz