system
Given the following post, respond as if you're commenting on Reddit:
human
Why does oil shine in all rainbows colours when it is in contact with water like rain?

I have seen it plenty of times when I have filled my car with gas.
gpt
It forms a very thin surface layer on the water. It's so thin that its thickness is in the order of magnitude of the wavelength of visible light. That in turn allows for interference between light reflected on the oil/air boundary with light reflected at the oil/water boundary. What wavelength constructively interferes is also dependent on the viewing angle. Since the surface you look at isn't perfectly flat, you see bands of multiple colours.