Why do we see colorful rainbow patterns in oil slicks on puddles? Can you explain the physics behind this everyday phenomenon?”

-Karthik Gurumurthy

I’ve always been fascinated by those rainbow-like colors that appear in oil slicks on puddles after it rains. The science behind this everyday phenomenon is actually quite beautiful.

Small amounts of oil are commonly present on road surfaces—lubricating oil from vehicles and bicycles. When it rains, these oil drops float on the water because oil is less dense than water (the same reason wood floats). Commercial oils typically contain surfactants that cause the oil to spread into a thin film on the water surface. This film is thickest at the center of the patch and gradually thins toward the edges.

What creates those vibrant colors is an optical effect called interference. Light reflects from both the top surface of the oil film and from the boundary between the oil and water beneath it. When these light rays reach your eye, the slight difference in their path lengths determines what you see. If the path length difference equals an integral multiple of the wavelength of a particular color, the rays will reinforce each other through constructive interference, making that color appear brighter. Other colors whose wavelengths don’t match up properly will undergo destructive interference and cancel out.

Since sunlight contains all colors of the rainbow, and each color has a different wavelength, specific path length differences will cause constructive interference for some colors and destructive interference for others. And because the oil film gradually varies in thickness from center to edge, different bands of the slick produce different colors, creating those stunning rainbow patterns we see on puddles.