Figure 1 – Iridescent metal, Natick, MA. (c) DE Wolf 2017.

It’s been quite a while since I’ve done a “reassuring moments in physics post.” So I’m offering up today Figure 1. I came upon these beautiful iridescent metal cups and flasks the other morning and fell in love with them photographically. There is something so, so appealing about iridescence – beautiful metallic colors shifts with your eye. The photograph required very minimal work-up. Look for me in the photograph. I am there in olive pants and pink shirt. But I am not the looming specter in the foreground. Whoever he is, he seems fittingly placed as we near Halloween.

Iridescence, a kind of metallic glow can be caused by several phenomena. Here it is the most common, thin film interference, much like the flash of color from an oil slick on a puddle. It is caused by a thin film of metal oxide on the metal surface. It is applied usually with a torch the heats-up and allows the metal to oxidize. A ray of light partially reflects off the film’s surface. The part that penetrates the oxide film reflects off the metal surface and rejoins the original ray, but it is delayed which causes its oscillations to be out of synch. It is said to be phase shifted, which causes it to interfere, like two waves at the beach. Some colors interfere constructively (intensifying them) others destructively (diminishing them) because the index of refraction of the film is wavelength dependent, causing the delay to be wavelength dependent. Remember that the speed of light through a material is the speed of light through a vacuum divided by the index of refraction. And the final part of magic is that as you shift your head and look at different angles the rays coming at you have longer or shorter paths through the film causing the colors to shift. This last part is really what we call the iridescence effect.

There you have it physics at the mall. Physics is everywhere – so reassuring!