A thin film of oil of thickness t is floating on water. The oil has index of refraction n_o = 1.4. There is air above the oil. When viewed from the air in the direction normal to the surface, there is constructive interference of reflected red light with l = 675 nm.

Find the minimum thickness t of the oil film that will give constructive interference of reflected light with l = 675 nm.

t_min = nm