Let us start from considering monochromatic light as an incidence on the film of a thickness t whose material has an index of refraction n determined by their respective properties.
From this point of view part of the light will be reflated and the other will be transmitted to the thin film. That additional distance traveled by the ray that was reflected from the bottom will be twice the thickness of the thin film at the point where the light strikes. Therefore, this relation of phase differences and additional distance can be expressed mathematically as

We are given the second smallest nonzero thickness at which destructive interference occurs.
This corresponds to, m = 2, therefore


The index of refraction of soap is given, then

Combining the results of all steps we get

Rearranging, we find



The resultant force is 5N.So the box moves to right with constant acceleration.
B. secondary waves aka shear waves
not too sure on this one maybe to locate objects under water, to diagnose problems, to treat medical ailments