1. The problem statement, all variables and given/known data White light illuminates an oil film on water. Viewing it directly from above, it looks red. Assume that the reflected red light has a wavelength of 615 nm in air, and that the oil has a thickness of 2.640e-7 m. What is the refractive index of the oil? Assume that the refractive index of water is greater than that of the oil. 2. Relevant equations 2nt = (m+1/2) λ 3. The attempt at a solution 2nt = (m+1/2) λ n = 1/2 λ/(2t) = 1/2 (615e-9)/(2*2.64e-7) n = 0.58239 I don't know why this answer is wrong. The problem seems to be straight forward, just plug in the numbers into formula right? If m is greater than 0 then the answer n comes out higher then the refraction index of water, 1.33, so m has to be 0 I presume? Edit: Never mind, figured it out.