1. The problem statement, all variables and given/known data Two pieces of optically flat glass (n=1.5) touch at one edge and are separated by a thin stretched fibre at the opposite edge. Light of wavelength 500nm is shone from above and 100 dark fringes are counted. The air between the glass is now replaced by water (n=1.33). How many dark fringes are now observed? a) 133 b) 75 c) 150 d) 100 e) 67 2. Relevant equations 2nt = (m+0.5)(lambda) .. I think? 3. The attempt at a solution I assumed that since the index of refraction for both water AND air are smaller than the index of refraction for glass, it does not make a difference. So I chose d)100. However, the answer is supposed to be a) 133.