# Why don't the colors in an oil slick change as I walk past?

After it rains, I often see bands of color on the asphalt roads.

This is usually explained as an thin film interference. Wikipedia has a picture of it which looks by inspection to be the same phenomena I'm seeing.

However, when I walk past these oil slicks, the colors appear to stay in the same place relative to the asphalt beneath them. As I walk past, I'm changing the angle of the incident and reflected light, so the colors I see at a particular spot on the oil slick ought to change.

What's going on?

• I think that it is basically due to the fact that the oil film is acting as a Fabry-Perot interferometer with a very small plate separation (i.e., the thickness of the oil film). The light effectively bounces between the oil film surfaces a finite number of times to create interference fringes. You can see that the total phase difference from first bounce to the last is relatively small in the case for something very thin like an oil film, and therefore the total change in this phase difference with angle tends to be small, too. In other words, you don't see much change when you walk past it.
– user93237
Apr 29 '16 at 20:30
• In your video, at 5s the purple contour mainly encloses the center of the spill; at 10s with the smallest angle of incidence it forms a figure-8, highlighting the wings; then at 13s it's restricted back to the center again. So I think you are actually illustrating the subtle variation with angle.
– pwf
May 10 '17 at 19:26

I did a little simulation using this software I wrote. The first box is air-oil-water at normal incidence, the second is looking at 45 degrees:

(Your youtube video is not truly air/oil/water, but rather air/oil/wet pavement, I think, which is a bit different. So don't take these pictures too literally. But they're illustrative.) The two images are only modestly different. It's only a modest change because, as @L. Levrel notes, the light bends towards the normal as it enters the oil. So the initial angle difference gets cut down.

But still, looking at these two plots, you might wonder: "Shouldn't this shift nevertheless be noticeable?"

Well here's the real answer. The change is even smaller than these two plots suggest because you're looking at a textured surface. For example, at the end of the video, you think you're looking at glancing angle, but you're not really. Most of your field-of-view consists of little pebbles or bumps sticking out of the pavement. And the surface of those pebbles is almost normal to your line of sight (as suggested by @Girish's answer and comments).

• Nice simulation! I believe the video is air-oil-water. This is why we only see these patches after it rains. May 17 '16 at 7:03
• Hmm, you're probably right. I changed the wording to "wet pavement" which presumably involves a microscopic film of water clinging to some or all of the irregular pavement surface. I don't really know. May 17 '16 at 11:40

I'll suppose you're familiar with the theory exposed in the Wikipedia page you're citing. Keeping its notation, path difference is proportional to $n_2\cos θ_2$.

But $θ_2$ is a result of refraction of incident light in air of index $n_1≈1$. So $\cos θ_2$ is bounded below: $$n_2\sin θ_2≤1$$ $$\sin θ_2≤1/n_2$$ $$\cos θ_2 ≥ \sqrt{n_2{}^2-1}/n_2$$

Taking $n_2$ to be at least $1.3$ for oil, you get $$\cos θ_2 ≥ 0.64$$

So the optical path difference varies by at most 36% when you change your position. This is hard to say from your video if fringes don't move at all, or move a bit.

• In the video though, I felt a rather wide range of angle of incidences were covered and the colors did'nt seem to change. A good test of your explanation could be to repeat this with oil slick in a flat water surface. May 14 '16 at 3:32

I suspect that it is due to the fact that the oil slick you observe has almost dried up on the road. The standard calculations assume a thin film of oil on a flat surface of water. Perhaps here the oil and moisture layers on the road take on the same deformations as that of the road. As a result, from whichever angle you look at it, the effective path length in the thin film remains the same.

• Even if the surface is corrugated, the viewing angle to a given point depends on your position with respect to it. Doesn't it? May 13 '16 at 20:05
• Imagine a spherical water droplet covered with a spherical shell of oil slick. In such a case, irrespective of the angle from which you view it, the path length difference remains the same. Perhaps the moisture and oil slick on the road surface are similar to the above situation in the sense that you can imagine the layer of water and the overlying layer of oil slick to exactly envelop the deformations on the road surface (essentially like adding a skin to the road). May 14 '16 at 3:39
• Owing to the fact that the road deformations are of length scales of millimeters, you can think of the road as a collection of spheres of those length scales, each covered with this two-layered skin of moisture and oil slick. May 14 '16 at 3:39