The rainbow colors in a petrol puddle on a rainy street

You’re dodging puddles on a grey Tuesday when you spot a proper psychedelic swirl in the gutter. It looks like a unicorn had a rough night, but it’s actually just light waves having a bit of a barney.

That petrol layer is thinner than a cigarette paper. When light hits it, some bounces off the top while the rest dives through to the water underneath and kicks back up.

These two reflections smash into each other like rival fans on derby day. Depending on the oil's thickness, they either cancel each other out or team up to blast those vibrant neon colors straight at your eyes.

Wait, how does the thickness actually decide which color we see?

Think of light colors like different sized blokes. Blue is a nippy little one with short strides, while red is a big lad taking long steps.

If the oil layer is just the right depth, the blue wave bounces back and aligns perfectly with its mate. If the gap changes even a tiny bit, blue gets out of sync and clobbers itself, letting the red wave take the lead instead.

Because the oil is spread unevenly—thicker in some bits, thinner in others—you get that shifting kaleidoscope effect rather than just one boring shade.

Hold up, how can light waves just clobber each other into nothingness?

Imagine two lads trying to push through a pub turnstile at the exact same time. If one is pushing while the other is pulling with the same force, nobody moves. They’ve completely cancelled each other out.

In the oil, if the 'peak' of the top reflection meets the 'trough' of the bottom one, they flatten out into a dead calm. It’s like two noises hitting your ears at once and resulting in total silence.

When a specific color gets clobbered like that, it leaves a gap in the white light. Your eyes then feast on whatever colors are left standing, which is why those trippy neons look so sharp.

Does that mean white light is actually a full house of every color?

Spot on. White light is basically a rowdy squad of every color in the rainbow all crammed into one bus. When they’re all together, they look like a boring, neutral blur.

But when that oil layer does its magic and 'clobbers' the green or the red waves, it’s like kicking a few players off the pitch. The ones left behind finally get some breathing room to show off.

That’s why those neons look so sharp. You’re not seeing a new color being made; you’re seeing the survivors of a light-wave scrap, standing out because their competition has been flattened.

So what happens if the oil is too thick to kick anyone off?

Spot on. If the oil is too thick, the light waves don't get a chance to have a scrap. They just bounce off the surface like a regular mirror, keeping the whole rowdy squad together.

Without that microscopic gap to trip them up, no colors get cancelled out. You lose the psychedelic neons and just get a boring, glare-heavy reflection of the sky.

It’s the thin film effect that does the heavy lifting. Once it gets too chunky, the physics gives up and you’re back to a plain old greasy puddle.