Hello, again a simple question: consider a mirror, with a transparent layer (index of refraction n) of certain thickness on it. If a ray is reflected by the mirror, while the mirror is moved normal to the plane, then one could observe a dopplershift. But what happens if the mirror is fixed, while the thickness of the transparent layer changes with time? I would equivalently expect a dopplershift, since the optical path length changes. But it's a little difficult to imagine, how the reflected wave form gets stretched/contracted.