Oil Film: Rainbow in a Puddle
Ziliang William Zhu
Award: Top 100
School: singapore american school
Teacher: dylan abbott
Category: Natural
Photo #21012
Oil Film: Rainbow in a Puddle
This photo shows a Lego oil well beside a puddle streaked with shifting rainbow colors. Oil films on water are famous for these iridescent, rainbow-like swirls: color that comes from thin air.
The colors appear because oil spreads across the water in an extremely thin layer or film, often only a few hundred nanometers thick! This results in a film whose thickness comes close to the wavelength of visible light itself.
The colors are produced by a phenomenon called thin-film interference. When light hits the thin film of oil that sits on the water, part of it reflects from the air to the oil surface while other parts pass through and reflect from the oil to the water surface. These two reflected waves travel different distances and then recombine.
When the path difference results in the waves of a particular color being in phase, the color gets intensified due to constructive interference, when they're out of phase, they cancel. Because of the film's varying thickness, the film will amplify different wavelengths, producing different colors such as blue, purple, yellow, and red.
One final step that seals the deal to create the beautiful rainbow we see is the light reflecting from the air to oil surface flipping by half a wavelength because oil has a higher refractive index, while oil to water reflection does not. That is what decides which colors show.
This is the same physics that color soap bubbles, showing how light behaves as a wave.
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