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Color and Light · entry 46 of 52
Oil on wet pavement shows a rainbow pattern for the same reason thin soap films do.
The longer version
A thin film of oil floating on water produces two reflected beams, one from the top of the oil and one from the oil-water boundary. Their path difference makes some wavelengths reinforce and others cancel, so white light breaks into bands of colour.
The effect is the same thin-film interference seen in soap bubbles, and gravity and spreading make the film uneven, which is why the bands form irregular swirls. Film thickness comparable to a wavelength of light is required, which is why a thick puddle looks plain. Mechanics and photographers both use the pattern as a clue to how thin a film is.
Where this entry sits
These facts explore how light behaves and how human eyes turn it into the colors we see. Along the way you meet rainbows, prisms, mirrors, shadows, lasers, and optical tricks that fool our vision every day. The full list sits on the Color and Light page, where every entry is listed in order.
More From Color and Light
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Read 48Butterfly wings often shimmer with structural color produced by tiny scales instead of dyes.
Read 49Blue pigments are rare in nature, so most blue animals rely on structure instead.
Read 50Cameras capture color by filtering light through red, green, and blue sensors.
Read 51Printed images use cyan, magenta, yellow, and black ink to reproduce many colors.
Read 52Stop signals are usually red because the color stands out against most natural backgrounds.
Read 1The human eye detects color using three types of cone cells sensitive to red, green, and blue light.
Read 2Rainbows form when sunlight bends and reflects inside countless tiny water droplets in the air.
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