>>11147253It's not just the thickness, it's also the angle you're viewing it from.
Basically if you look straight into the oil - path is short
if you look at an angle, path through oil is longer.
If the thickness, taking angle into account, matches whole number of half-wavelengths (because light goes first down then up), then it will be strongly reflected. If it matches a quarter wavelength + whole number of half wavelengths, then it will almost not reflect at all.
