1. The problem statement, all variables and given/known data Light of free space wavelength 600 nm (6000 Å) travels 1.6 10 -6 m in a medium of index of refraction 1.5. Find: 1. the optical path length, [This is the physical path length x refractive index.] 2. the wavelength in the medium, and 3. the phase difference after moving that distance, with respect to light travelling the same distance in free space. 2. Relevant equations 3. The attempt at a solution just for part 3 I got the right answer but Im just a little confused if my resoning is right. the phase difference I calculated using d∏/λ I used Pi since the refractive index was not an integer so I thought it would be destructive intefeerence. The answer comes out to be 8∏/3 but I am a little confused on why?