Petrol rainbows

Sunlight comprises light of a range of wavelengths that span the visual spectrum. Consider for a moment monochromatic light of just one wavelength. When this light hits the surface of an oil slick, some of it will reflect off the surface and some of it will penetrate the slick, reflect off the surface beneath, and exit the slick again. When the bit of light that reflects off the slick rejoins the bit that bounces off the surface, the two will be out of phase due to the extra distance travelled by the bit that reflects off the surface. For instance, if the monochromatic light is blue, we could observe anything from bright blue light shining off the surface of the slick (when the two bundles of light are perfectly in phase), to no blue light (when the two bundles of light are perfectly out of phase), depending on the depth of the oil slick.

Because different colors of light have different wavelengths, we expect two different colors to have different brightnesses based on the local depth of the oil slick. As long as there are slight variations in the depth of the slick, we expect intense colors (less interference) to dominate weaker color (higher interference) and for the identity of these dominating colors to vary over the slick.

As a result, if the depth of the slick varies slowly, we should expect a smooth gradient of colors. If the depth varies quickly, we expect many cycles of periodic variation in colors over the slick.

it’s just a spectacular combination of oil, water and light rays.small patches of rainbow colors on a road on a sunny day, you are actually witnessing a striking display of reflection and refraction at the same time!This phenomenon occurs because when it rains, this oil comes (on the road) in contact with water. As you’ve surely heard in the past, oil and water do not mix, because the density of oil is less than that of water. The thickness of the oil layer also varies, from being as thin as a single molecule to extremely thick. Therefore, an oil layer of varying thickness floats atop the layer of water covering the road.

Effectively, there are three media that light rays come in contact with; air, oil and water When light rays strike the top surface of the oil film, some of them are reflected immediately, while others are refracted (i.e., they travel through the layer of oil and are subsequently reflected by the bottom surface of the oil film). In other words, some of the light rays traveled less of a distance than others to reach your (the observer’s eyes), so there is a difference in the length of the path.

If the difference in the length of the paths traveled by the light is an integral multiple of the wavelength of light, then rays reflected from the two spots reinforce each other and cause constructive interference, whereas if the rays cancel each other out, then destructive interference takes place.

In simple, non-technical words, due to the difference in the thickness of the oil film, light rays have to travel different distances, and they then mix together after reflecting to produce a spectrum of colors.

This is called the thin-film interference .