What is the thinnest film of MgF2 (n = 1.39) on glass that produces a strong reflection for light with a wavelength of 680 nm? This is the exact question I am faced with. I didn't really know where to begin so I tried using the ratio of n-air/wavelength to n-film/(new wavelength) to try and find the original wavelength. I found that, and really didn't know where to go then. I have looked around, and I just can't seem to find anything. Any help would be greatly appreciated.