I have an interest in coin toning, and particularly like the spectrum of colors displayed over the surface of a coin. In my effort to understand this effect better, I did some research and have compiled the following explanation. It is by no means exhaustive...feel free to add your comments where I can improve the science behind my explanation.

Constructive and destructive interference of light waves is the reason why thin films, such as oil films, soap bubbles, or patina on metals and glass show colorful patterns. This is known as thin-film interference, because it is the interference of light waves reflecting off the top surface of a film with the waves reflecting from the bottom surface. To obtain a nice colored pattern, the thickness of the film has to be on the order of the wavelength of light.




In this example, light reflecting from the bottom surface travels an extra distance of half the wavelength of green light. Green light reflected from the top surface interferes destructively with green light from the bottom surface; in other words, all the green light and most of the light in the middle of the visible spectrum, is transmitted. Some red and violet light is reflected, so the patina looks purple.

The "color" of the toning is directly due to the thickness of oxidation on the surface, and over time these colors will change as the coin continues to oxidize. That is something to consider if you spend the extra money for a particular toning effect.

Intersting. It could use more details. I could use this as an article on my website with a little more info added. Interested?

Thanks...I wanted to initially keep it very general w/o including terms like "phase shift", nanometers, electromagnetic wavelengths and the like.
Agreed regarding the diagram--I could certainly include more detail that may or may not interest the coin collector. Perhaps the diagram could use a simple schematic of destructive interference? I find the most interesting aspect of coin toning is how the thickness of oxidation determines the color effect over color absorption/reflectance of the base metal. I've noted some confusion among collectors who attribute different colors to various causes, when it's one single principle at work.

This was a quick writeup over lunch. Whenever I find time to improve this writeup, I'll let you know....thanks again!

Very interesting. Thanks

Sweet! Thanks Kurt! I've always wondered.
But I agree, my inner geek is wanting more!
Cheers,
M

Thanks....if you Google "thin-film interference" you'll find more info that you probably ever need!

Looks good, Kurt, from the POV of someone who may very well end up with a PhD in thin films.

Hmm... the Science of Coin Collecting... anyone think we could start a subforum on the subject and eventually push it into publication?

More physics! I love it!