Skip to Content

What is the rainbow in oil called?

When light passes through a transparent medium like glass, water or oil, it can be refracted and separated into its different wavelengths or colors. This separation of white light into its rainbow spectrum is called dispersion. The colorful patterns and shapes that result from this dispersion of light through oil are often referred to as oil sheen, iridescence, or in some cases, rainbow sheen.

What Causes Rainbow Colors in Oil?

The rainbow effect seen in oil slicks and thin films of oil on water, pavements, or other surfaces is caused by the refraction and reflection of light passing through the oil layer. Here’s a more in-depth look at how it works:

  • When white light (which contains all the colors of the visible spectrum) hits the top surface of the oil, some of it is reflected off and some enters the oil.
  • As the light passes into the oil, it slows down because oil is more optically dense than air.
  • This change in speed of the light causes it to refract or bend at the oil-air boundary. Different wavelengths (colors) bend by different amounts based on their frequency.
  • The dispersed colored light is then reflected off the bottom surface of the oil layer and recombines when passing back out into the air.
  • But because the colors were separated as they passed through the oil, they reflect and refract at different angles, creating a rainbow effect to our eyes.

The thickness and consistency of the oil determines how pronounced this dispersion effect will be. Thinner oil layers will show more vivid rainbow colors, while thicker layers appear darker brown or black since more light is absorbed. The specific composition of the oil also impacts the interaction with light waves and the resultant color patterns seen.

Common Examples of Rainbow Reflections in Oil

Some common examples where you can observe this optical phenomena of rainbows in oil include:

  • Oil slicks – Iridescent swirls and patches floating on the surface of water, caused by leaks or spills of crude oil or petroleum products.
  • Gasoline/oil puddles – On pavement, concrete or soil surfaces, a spilled puddle of gasoline, motor oil or other refined oils can create vibrant color effects as light passes through.
  • Soap bubbles – A thin film of soap solution produces rainbow reflection, this effect is often enhanced by adding glycerin to increase viscosity.
  • Oil-coated bubbles – Blowing bubbles and capturing a thin oil layer on their surface creates stunning rainbow bubble effects.
  • Motor oil – The iridescent sheen visible when light hits a puddle or film of motor oil on a flat surface.

In each of these situations, the key factors are having a thin film or layer of oil through which light can pass and disperse into spectral colors.

Other Terms Used to Describe Rainbow Oil Colors

There are a few other scientific terms used to describe the vivid colorful reflections seen in oil slicks and thin oil films:

  • Iridescence – The general term for the shimmering rainbow-like colors caused by diffraction and interference of light waves.
  • Schlieren effect – The optical effect caused by variations in refractive index appearing as colored patterns and textures.
  • Thin-film interference – Alternating light and dark bands produced when light reflects off the top and bottom of a thin film.
  • Diffraction gradient – The change in diffraction across a fluid layer like oil due to differences in thickness.

While oil sheen, rainbow sheen, and iridescence are more colloquial terms, schlieren patterns, thin-film interference and diffraction gradient are technical terms used in optics and physics to explain the light phenomena.

Color Patterns and Textures Seen in Rainbow Oil

The exact colors and patterns visible in a rainbow oil effect can vary considerably based on the oil thickness, composition, viewing angle and lighting conditions. Here are some of the typical features you may notice:

  • Swirls – Twisted, intermixing bands of color caused by turbulence in the oil layer.
  • Wavy lines – Caused by variations in oil thickness showing alternating colors and light/dark interference bands.
  • Spots and dots – Small rounded color patches caused by diffraction through irregular oil globs.
  • Sheen – Glossy shine with metallic hues like purple, green and gold.
  • 3D texture – The thick and thin regions can give a textured, almost topographical appearance.

When viewed up close, these complex mixes of color, bands, dots, swirls and textures can appear almost pixellated or impressionistic in quality. The constantly shifting patterns contribute to the mesmerizing visual effect.

Why Does Oil Create Rainbow Colors?

We see rainbows in oil because of the unique way oil interacts with light compared to air or water. Here are some of the key properties of oil that give rise to rainbow reflections:

  • Viscosity – Oil has a thicker, more viscous consistency than air or water. This slows down light passing through it and enhances dispersion.
  • Refraction – Oil has a higher refractive index than air, causing more distinct bending and separation of colors.
  • thin film – Oil tends to spread into a thin layer only microns thick, ideal for interference colors.
  • reflective – Oil is partially reflective, causing the separated light to bounce off its top and bottom surfaces.

The combination of oil’s viscosity, refractive index, spreading behavior and reflectivity make it the perfect medium for splitting white light into a rainbow spectacle of colors through diffraction, refraction, reflection and interference.

Rainbow Effect in Other Liquids Besides Oil

While oil is the most common liquid we observe rainbow iridescence in, other liquids can also generate similar colorful interference patterns when in a thin film. Some examples include:

  • Gasoline – The iridescent sheens swirling in gasoline puddles are caused by its high volatility and intermediate viscosity.
  • Soap bubbles – Interference colors form on the thin soap film on the bubble surface.
  • Antifreeze – The glycol compounds produce a refractive index gradient that splits light.
  • Motor oils – Synthetic and mineral motor oils can create vibrant rainbow effects when spilled or smeared.
  • Paint – Certain thin paints and varnishes give off an iridescent sheen.

However, the unique chemical makeup of crude oil and petroleum products make them most predisposed to generating vivid spectral colors with even small amounts spilled into thin surface films.

Interesting Facts About Rainbow Oil Sheen

Here are some fascinating facts and notable points about the rainbow sheen we observe in oil:

  • Crude oil is a complex mixture of over 17,000 distinct chemical compounds, each with their own light interacting properties.
  • The colors arise from interference of reflected light, not direct pigments like paints or dyes.
  • Rainbow sheen can indicate the presence of oil pollution in water that may not be visible otherwise.
  • Oil thickness affects color intensity – thinner films give more vivid colors.
  • The rainbow sheen is caused by only the top few microns of the oil film.
  • Viewing angle impacts the color pattern – it shifts as your perspective changes.
  • Temperature affects oil viscosity and refractive index, changing the observed colors.
  • Surfactants like detergents can dampen the rainbow effect by changing the oil-water interface.

So in summary, the vivid colors are produced entirely through the physical interaction of light with the complex chemistry of oil without requiring pigments or dyes.

Benefits and Uses of Rainbow Oil Optics

While rainbow oil sheens are often seen as pollution, their unique optical properties can be harnessed for creative purposes like:

  • Oil-based projection light shows and stage effects
  • Making stunning rainbow bubble solutions
  • Creating color-changing inks, paints and cosmetics
  • Educational light physics demonstrations
  • Artistic photography exploiting oil iridescence
  • Detecting and visualizing small oil spills through sheen
  • Spectroscopy calibration standards for identifying oil composition

With some innovative thinking, we can find positive applications for the colorful optical phenomena in oils.


In summary, the shimmering rainbow colors seen swirling in oil slicks and thin films are produced through the diffraction, dispersion, interference and refraction of light passing through the oil. While often referred to as oil sheen, iridescence, or rainbow sheen, the technical terms are thin-film interference and diffraction gradients. The effect arises from oil’s viscosity, refractive index and reflective properties interacting with light. While mesmerizing, rainbow oil sheens can indicate the presence of pollution. But their unique optics can also be harnessed for creative purposes and to detect or analyze oil spills. So next time you spot a rainbow swirl of colors in an oil puddle, you’ll know it’s caused by the complex dance of light at the oil-air interface.