You open a package of sliced ham or roast beef and suddenly notice something strange: a rainbow-like shine flashing across the surface. Green, blue, purple, or gold colors seem to appear depending on the angle of the light. It can look alarming enough to make you wonder whether the meat is spoiled or covered in some mysterious chemical.

In many cases, though, that colorful sheen is simply an optical effect. Thinly sliced meat has a very organized microscopic structure, and when light hits the cut muscle fibers, some wavelengths can be reflected differently from others. That can create an iridescent appearance similar to what you see on soap bubbles or certain shells. But the rainbow itself isn’t the most important thing to check before deciding whether the meat is safe to eat.