Have you ever gotten a piece of roast beef that looked like it might be starting to go bad because it had an iridescent sheen on it that looked like something you'd see on the side of a fish?
That metallic shine may have put you off and caused you to throw away perfectly edible roast beef just to be safe, but as it turns out that metallic shine has nothing to do with the meat being old.
Why does this slice of roast beef have metallic sheen pic.twitter.com/m2RvAJ00Iw— 360 nohope hiatus (@_ryugazakis) March 30, 2016
In fact, the iridescence is actually caused by an optical phenomenon called interference:
Optical interference happens whenever a light wave reflected off one surface encounters a wave reflected off another surface close by.
In the case of lunchmeat, two sorts of interference effects are possible.
The first is called thin-film interference. Some deli cuts, especially cured meats, are rich in fat and oil. If this fat seeps out, it can form a layer on the surface and change the situation from a simple single reflection to a double reflection—one off the front surface where the air meets the fat layer and another off the back surface where the fat layer meets the meat.
This type of interference is what gives oil slicks their hues of purple and green, and what gives bubbles that warbling rainbow shimmer.
The second interference effect is called diffraction. Diffraction occurs whenever light passes through a repeating grid of equally-spaced slits or bounces off a surface of equally-spaced reflectors. This structure—called a diffraction grating—produces many reflection points, meaning that instead of considering how just two reflected waves interfere, we must consider the sum of a great multiplicity.
What does this have to do with lunchmeat? Muscles are made up of proteins that bind into strands, and these strands in turn group into long fibers, each about one or two micrometers in diameter.
When a muscle is cooked and ultimately cut, this repeating structure of muscle fiber is exposed, forming a natural diffraction grating for visible light. Any mixture of light incident on the slice of meat will encounter these well-ordered fibers and reflect only certain colors out at certain angles.