A thin lay of oil floats on the water in a wet parking lot. The index of refraction of the oil is 1.55 and water is 1.33 What is the minimum thickness of the oil for light at normal incidence to be maximally reflected from thin film interference for Blue light at 450nm. (sunlight at noon for example)

Later in the day (~6pm in summer) light is incident at 45 degrees. What wavelength would be maximally reflected at this angle assuming the same thickness as above.