An oil slick is like an uneven sheet of glass. Colors are seen because red light is slower than blue light. The human eye sees light as white unless something alters how the light arrives at our eyes. So, in effect, the different colors reach our eyes at different times, depending on where we are looking. Now let's look at another property of light.
Light is reflected from all points on a mirror , as shown in the diagram that follows.
If it were possible to use a stopwatch to time the movement of photons from the source to the detector, the times near the center would be the shortest, and they would be nearly equal. Because the light from the center strikes our eyes first, that is the reflection that we see. The variations in speed at the ends is greater, so the hand on the stopwatch would show a greater variation. We don t see this light, because the photons tend to cancel each other out. The hand of our imaginary stopwatch would look something like the following for various spots on the mirror.