After a rain, one can often see patterns when a little oil floats on top of puddles. The colors will be in the order: red, orange, yellow, green, blue, and violet. Each of these colors of light has its own wavelength, and different parts of the oil have different thicknesses. Part of the light from the sun bounces off the top surface, in other words it bounces off the oil. Part of the sunlight bounces off the top of the water. The light waves from the oil surface and the light from the water surface meet back in the air, and they interfere. For any thickness of the oil layer, some of the light waves will add and some will subtract, so the result is that one color of light will be strongest there.