What causes the process called interference?

Answer 1

Due to superposition principle.

What is it?

When two or more waves traverse in a medium, they travel as if the other waves were absent. What does it mean?

In case of marbles moving across if they come into contact they would collide and change it path. But as the waves meet together no such material collision and no change in the direction of their motion. Hence that valid statement as the first part of superposition principle.

Now second part is for the interference part. Though there is no collision, but at a particular point in the medium the displacement of the particle in the medium is the resultant of the displacements produced by all the waves at the instant. So if the resultant displacement is minimum then it is termed as destructive interference and if the displacement is maximum then it is considered as the constructive interference.