A laboratory dish, \(20 \mathrm{~cm}\) in diameter, is half filled with V. water. One at a time, \(0.50 \mu \mathrm{L}\) drops of oil from a micropipette are dropped onto the surface of the water, where they spread out into a uniform thin film. After the first drop is added, the intensity of \(600 \mathrm{~nm}\) light reflected from the surface is very low. As more drops are added, the reflected intensity increases, then decreases again to a minimum after a total of 13 drops have been added. What is the index of refraction of the oil?