Rainbows spilled on the pavement

One day wandering home down along usual streets. Then I noticed: a rainbow spilled on the pavement.
Unlike its heavenly counterpart, not a simple geometric figure resembling a ring.
More like a kaleidoscopic chaos of patterns intertwined, spectral splatter...

Why oil on water looks like a rainbow..?

Oil and water will not mix. Drops of oil float on water, spreading wider and wider, thinner, thinner.
Soon so thin that waves of light interfere with themselves inside.

Light is partly reflected both from the top (oil/air) and bottom surface (water/oil).

First some light is reflected from the top, back into our eyes. The rest continues inside.
Some is then reflected up from the bottom (the rest disappears into water), to meet the top again.
Some continues up to reach our eyes, but the rest is reflected downward for the second time, thus iterating the whole process...

Each round intensity drops and eventually fades out. Meanwhile all these light waves interferes internally,
creating rainbow cascades of spectral mutations. Notice that colors shift, if your eyes move.
These hues are more related to light than matter...



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