1. The problem statement, all variables and given/known data So if light travels from vacuum to oil to water (1 -> 1.4 -> 1.33) how do you know if there is a constructive or destructive interference? 2. Relevant equations 3. The attempt at a solution I know that when it goes from air to the oil there is a phase shift of 1/2. But when it goes from oil to water, there is no phase shift. How would you determine if the entire thing is constructive or destructive or not? Would you add the two phase shifts together to determine it?