plz help me again(phy) follow up?

A phase change of pi radians (or 入/2) occurs when light reflects from a less dense medium to a denser medium.

In the problem, the first light ray is reflected on the surface of the coating, which is from a less dense medium (air of refractive index 1) to a denser medium (coating of refractive index 1.4). There is a phase change of pi radians (or 入/2).

The second light ray is reflected on the glass surface, which is from a less dense medium (coating of refractive index 1.4) to a denser medium (glass of refractive index 1.5). There is a phase change of pi radians (or 入/2).

Since there is a change of pi radians (or 入/2) for both reflected light rays, the effect of phase change cancels out each other. Hence, a path difference of 200入 implies that there is constructive interference.