In a laboratory accident, you spill two liquids onto water, neither of which mixes with it. They both form thin films on the water surface. As the films spread and become thin, you notice that one film becomes bright and the other black in reflected light. Why might this be?
When two waves going in the same direction have the same phase or phase difference, constructive interference occurs and they give a bright fringe or zone. If they are of opposite phases, they result in destructive interference and cause darkness. The same event occurs for waves reflected from the upper a lower layer of the oil films
For some films (which are not uniform), the thickness might be equal to or an integral multiple of a certain wavelength present (depending on the thicknesses), which causes constructive interference for that wavelength and so result in brightness. If the thickness is equal everywhere, this occurs for only one wavelength giving only one color. But if not so, some portion will fulfill such condition for blue, some for red, some for violet, and so on. This collectively results in white reflection So, the layer appears white and shiny.
For other films, the thickness will not be even near the integral multiples of any wavelength, causing destructive interference and so total darkness. So such films appear black.
