1. The problem statement, all variables and given/known data A light wave is propagated from point A to point B in space. We introduce along the way a glass lens with parallel faces of index 1.5 and width L=1mm. The value of teh wavelength is 500μm in space. How many wavelengths are between A and B with and without the glass lens? What is the phase difference introduced as we intercalate the glass lens? 2. Relevant equations f=v/lambda v=c/n 3. The attempt at a solution With the lens, I just used f=v/lambda which gave me f = 6x10^14 Hz. Without the lens, I first used v = c/n and then used v in f=v/lambda to get 4x10^14 Hz. I hope I atleast got the first part right. I'm not sure how to go about finding the difference in phase. Any pointers?