Question

how are shadows formed?

Answer 1

Basically an object that does not allow any light to pass through will form shadow. Shadows are formed by light because if light shines on to an object and the object blocks the light, the light will go sideways of the object and therefore a shadow is formed.
Hope it helps.

Answer 2

SHADOW IS FORMED WHEN THE LIGHT IS TRAVELLING STRAIGHT AND IT IS BLOCKED BY AN OBJECT THAT DOESN'T ALLOW LIGHT TO PASS THROUGH IT AND THE LIGHT WILL GO SIDEWAYS AND IT WILL FORM AN SHADOW. THE OBJECT SHOULD BE OPAQUE OR TRASULANT THAN ONLY SHADOW WILL FORM OTHERWISE IT DOES NOT FORM SHADOW.