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a film of oil on water

# a film of oil on water - a film of oil on water Working...

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a film of oil on water Working through an example is a good way to see how the step-by-step approach is applied. In this case, white light in air shines on an oil film that floats on water. When looking straight down at the film, the reflected light is red, with a wavelength of 636 nm. What is the minimum possible thickness of the film? Step 1. Because oil has a higher index of refraction than air, the wave reflecting off the top surface of the film is shifted by half a wavelength. Step 2. Because water has a lower index of refraction than oil, the wave reflecting off the bottom surface of the film does not have a half-wavelength shift, but it does travel the extra distance of 2t. Step 3. The relative shift is thus: Step 4. Now, is this constructive interference or destructive interference? Because the film looks red, there is constructive interference taking place for the red light. Step 5. Moving all factors of the wavelength to the right side of the equation gives: Note that this looks like an equation for destructive interference! It isn't, because we

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