Assume that light is incident normal to the surface of a film of thickness ddd. How much farther does the light reflected from the back surface travel than the light reflected from the front surface

The answer is 4f======since electrostatic force is given by    we let f be the electrostatic force when  r = 1 m    if we let  denote the electrostatic force when the distance is decreased so that r = 0.5m, then    we established earlier that f = kq₁q₂. therefore    the force will now be 4f.