Subtraction: If a specific charge is surrounded by multiple charges, then it is necessary to find the working net force on that charge. To find the net force, one has to determine the vector sum of the force applied to the charge considered by each charge individually. This is called the overlapping principle of an electric ball. Refraction: When a ray of light penetrates a sharp edge, a play of a faint light and shadow is seen a little before the path on which it is supposed to go on the screen , this is refraction.