I feel really lost in this interference stuff. I'm also having trouble with the following problem: A mixture of yellow light (wavelength = 584 nm in vacuum) and violet light (wavelength = 408 nm in vacuum) falls perpendicularly on a film of gasoline that is floating on a puddle of water. For both wavelengths, the refractive index of gasoline is n = 1.40 and that of water is n = 1.33. What is the minimum nonzero thickness of the film in a spot that looks the following colors because of destructive interference? What makes matters worse for this problem is there is even an example of this type of problem in my book. However, I used the formula wavelengthfilm = wavelengthvacuum / n and figured out that the wavelength of yellow in the gasoline is 417 nm and for violet is 291 nm. The problem also said that the color is caused by destructive interference. So in my mind this would mean that 2t + 1/2 wavelengthfilm = conditions for destructive wavelenth. I figured it was 2t since wave 2 traveled extra distance. In the end this would mean that for a miniumum nonzero thickness of film my formula would equal t= 1/2 * wavelength film. However webbassign is saying my answers are incorrect. Any help would be appreciated.