I have run into a problem and I haven't been able to even begin figuring it out: A point source of light with wavelength 589 nm is placed 0.4 mm above a mirror. Interference fringes are observed on a screen 6 m away, and the interface is between the light coming from the source and the light reflected off the mirror. Find the spacing of the fringes. -I know that fringes will occur every time the phase changes by 2 pi, and that in this case the phase change of the reflected light will be: (difference of path)(2 pi)/wavelength but I'm not sure if reflecting off the mirror causes a pi phase change cause the index of refraction of the mirror is not given, so I can't tell if it is greater than the index of air. But my real problem is finding the path difference and relating it to the screen that is 6m away. Any hints on where to begin?