Light · fluids · about 12 minutes
Why does oil on water make a rainbow?
Motor oil is brown. Gasoline is pale yellow. Water is clear. Yet a few drops of either on a wet road turn into swirling purple, gold, blue and green. There is no dye in there. So where do the colours come from?
Every colour you see here is computed from one thing only: how thick the oil is at that spot. The thick middle of a fresh drop is brown, the colours appear as it spreads thinner, and the thinnest edges fade to grey. By the end of this page you'll know why.
Part 1 · Why it sits on top
Floats, sinks, or vanishes?
Before oil can make colours, it has to stay in a thin layer on top of the water. That needs two things to be true at once. Find out which two by pouring five liquids into a jar of water.
For each liquid, predict first, then pour. Try all five.
Pick a liquid.
The two rules
1. It must not mix. Rubbing alcohol is lighter than water too, yet it disappears into it. Water molecules grip each other tightly. Alcohol molecules can grip water back, so they slip right in. Oil molecules are long chains with nothing to grip with, so the water holds on to itself and pushes the oil out.
2. It must be less dense. Once the oil is pushed out, density decides where it goes. Gasoline (0.74 g/cm³) and motor oil (about 0.87 g/cm³) are lighter than water (1.00 g/cm³), so they rise and float. Corn syrup (1.38 g/cm³) is heavier, so it sinks.
One twist: the heaviest fuel oils carried by ships are about 0.99 g/cm³. They barely float in fresh water, float better in salty sea water, and can sink once they pick up sand. That makes those spills much harder to clean.
Part 2 · How thin it gets
Spread one teaspoon of oil
Oil on water doesn't stay as a drop. It keeps spreading out, getting thinner. In 1773 Benjamin Franklin poured one teaspoon of olive oil onto a pond in London and watched it spread across about half an acre, calming the ripples. Spread the same 5 mL over bigger and bigger areas and watch what happens to its colour.
5 mL ÷ 5 m² = 0.000001 m
rainbow
The ruler is logarithmic: every step is ten times thinner. The purple band is how long a wave of visible light is, from violet (0.4 µm) to red (0.7 µm).
Find the area where the colours show up, then compare the film with the purple band on the ruler. What do you notice?
A rainbow film is about as thick as one wave of light is long. That is the clue. Franklin's pond film was thinner still, about two millionths of a millimetre, roughly one molecule thick. In 1890 Lord Rayleigh used exactly this trick to make one of the first estimates of how big a molecule is. Motor oil and gasoline stop spreading long before that, so a fuel spill lingers in the rainbow zone.
Part 3 · Two echoes
One beam in, two bounces out
When light hits the film, a little of it bounces off the top surface (air to oil). The rest goes into the oil, and a little bounces off the bottom surface (oil to water). So your eye gets two echoes of the same light. The bottom echo has made an extra trip down and back through the oil, so it arrives a bit behind.
1. With red light, find a thickness where the two echoes line up and red comes back bright.
2. Still red: find a thickness (thicker than 100 nm) where red cancels out.
3. Find a thickness where red is gone but blue is bright. What colour would that film look?
Why is the thinnest film dark for every colour?
The top bounce happens where light goes from air into something slower (oil), and that flips the wave upside down. The bottom bounce goes from oil into water, which is a little faster than oil, and doesn't flip. With almost no oil, there's almost no extra trip, so the two echoes are exactly opposite and cancel. That's why the very thin edge of a slick looks dull and grey, and why a soap bubble turns black just before it pops.
In real oil the bottom echo is weaker than the top one, so a colour never cancels completely. It just dims. That's why oil-film colours look soft and shimmery instead of neon. The waves here are drawn equal so you can see the cancelling clearly.
Part 4 · White light in, colour out
Every colour at once
Sunlight has all the colours in it. At any one thickness, some colours line up and come back strong, while others cancel. Your eye mixes what's left. Take away blue and the mix looks gold. Take away green and it looks magenta.
Tap the long strip to jump to a thickness. That strip is the whole story of the puddle: every thickness from 0 to 1.5 µm and the colour it makes.
Paint by thickness: pick a target, then slide until your film matches it.
Part 5 · Tilt your head
Why the colours slide as you walk by
Walk past a puddle and the bands of colour seem to crawl across it. The oil hasn't moved. You're looking from a different angle. Below is a strip of oil that gets thicker from left to right, seen from straight above (bottom strip) and from your angle (top strip).
Predict: if you look from low down, does the marked band slide toward the thick end or the thin end?
Make a prediction, then tilt to at least 60° to check it.
Seen at a slant, light runs through the film at a slant too. You might expect a longer trip, but what matters is how far the bottom echo lags behind the top one, and that lag gets shorter: it's 2 × 1.47 × thickness × cos(angle inside the oil). A shorter lag means each colour now needs thicker oil to line up, so every band slides toward the thick end, and colours shift toward the blue end of the rainbow.
Part 6 · Read a real spill
Colour is a thickness gauge
Spill responders flying over the sea use exactly this physics backwards. From the colour they estimate how thick the oil is, then multiply by the area to estimate how much was spilled. This is the field guide they use (the Bonn Agreement oil appearance code), simplified:
| Looks like | Name | Thickness |
|---|
Name each patch from the air.
A rainbow slick covers 1 square kilometre of sea, about 1 µm thick. Roughly how much oil is that?
Put it back together
The rainbow in three steps
The colour isn't in the oil. It's made by the film's thickness. That's also why soap bubbles swirl with colour, why the colours change when you tilt your head, and why the brown middle of a fresh drip shows no rainbow at all: it's still too thick.