I use a stainless steel saucepan for cooking pasta. Sometimes after washing it, there’s a pale rainbow sheen on the inside bottom surface. Questions:
What am I doing to make this happen? Washing it while it’s still hot? Boiling salted water?
What causes the rainbow effect? Diffraction, presumably, but how and why?
Google turned up a bunch of contradictory theories.
Is it like an oil slick? The rainbow effect is similar to bubbles - caused by a layer about the same thickness as the wavelength of light (0.4-0.7 micrometers). Light is reflected from the front of the layer, and some passes through the layer, reflects off the back, passes through the layer again and combines with the light from the front. The interference cancels out some colors, leaving others.
It’s going to be something like this. The layer itself might actually be some sort of spontaneously-formed anodization
I’ve always heard it was from overheating the pan. Most of the links that came up for me on Google were similar to this, from Stellar Cookware:
I know that when I use my pans they’re fine, but when my husband uses them the rainbows appear. 
A similar thing happens on other metal surfaces when they’re heated (for example, chisels if they’re allowed to overheat through friction with a rotary grindstone. The coloration is likely to be a thin, transparent oxide layer, that happens to be of a suitable thickness for optical interference to occur.
I own a set of stainless steel cookware that always does that after I cook with it. The salesperson who sold me the cookware said that’s normal for stainless steel, and it’s not toxic or anything.
A popular product to remove the “rainbow” is Barkeeper’s Friend.
Interesting! I guess that’s why I was told never to use my pans on anything higher than medium heat. Although I still get those rainbows…
I think they’re also caused by high electric currents (which produce lots of heat), but can’t find any pictures right now.
Is the rainbow a temporary effect? And Do the colors change with angle?If so, it’s probably a thin layer of water or soap causing thin-film interference, like the above-mentioned oil slicks.
If they don’t go away and persist when the pan in dry, but change with angle, then it might be the formation of a thin oxide layer, as noted above.
Rainbows that remain after the pan is dry and don’t alter color with angle are probably some chemical change, probably non-interference oxide layers.
It’s there when the pot is dry (in fact it’s invisible when the surface is still even slightly wet). Scrubbing with steel wool takes it off. A non-scratch scourer doesn’t. An oxide layer sounds plausible.
I wouldn’t say the colours change, but it does get less visible at an acute angle of view. Hard to say for sure, there are reflections from the inside walls that confuse things.
I’ve heard it can be caused by overheating too, or by heating it while empty. I’m careful not to do that. But I do sometimes wash this particular pot while it’s still hot.
I’m not concerned it’s harmful, just curious about what’s going on.
Yup, now that you mention it I’d say that’s the same effect.
I’m pretty sure I haven’t overheated this pot; would cooling it quickly do that? Washing it in warm water while it’s still hot.
I have no financial interest in Barkeeper’s Friend or Bon Ami, but I’ve found that Bon Ami works better.