Question

how is shadow formed

Answer 1

Shadow needs two things to form:

1.Light.
2.Object.

Basically an object that does not allows any light to pass through will form a shadow.
Shadows are formed by light, because if light shines onto an object, and the object blocks the light, the light will go sideways of the object, and therefore, a shadow is formed.
Light can only travel in straight lines. If light is coming from one main source, then an object that does not allow light to pass through it, will have an absence of light behind it, i.e.a shadow.

Answer 2

when the light falls on object the shadow is formed.