1. The problem statement, all variables and given/known data A double-slit experiment uses a helium-neon laser with a wavelength of 633 nm and a slit separation of 12mm. When a thin sheet of glass is placed in front of one of the slits, the interference pattern shifts by 5 fringes. When the experiment is repeated under water, the shift is only 3 fringes. If the the refractive index for the water-air interface is 1.33, find: a) the thickness of the glass sheet b) its refractive index 2. Relevant equations None 3. The attempt at a solution I understand that the light beam is being diffracted through the sheet of glass, thus having to travel a longer path and that's why the interference pattern is shifted, but I don't know how to formulate the whole problem. Help?