A scientist notices that an oil slick floating on water when viewed from above has many dissimilar rainbow colours reflecting off the surface. She aims a spectrometer at a particular spot and measures the wavelength to be 750 {\rm nm} (in air). The index of refraction of water is 1.33.
1. The index of refraction of the oil is 1.20. What is the minimum thickness t of the oil slick at that spot?
2. Assume the oil had an index of refraction of 1.50. What would the minimum thickness t be now?