Hi i was going through some of my physics past papers to prepare for exams, when i came across this question A film of oil of refractive index 1.45 rests on a flat piece of glass of refractive index 1.6. When illuminated by white light at normal incidence, light of wavelengths 690 nm and 460 nm are predominant in the reflected light. I dont understand this phenomena. I tried to explain it through dispersion, but dont think dispersion can happen at normal incidence. Would Some one please explain the physics behind this.