If Your Ham Has a Rainbow Sheen, This Is Why

Your iridescent deli meat, explained.

Deli meat is a lifesaver. It’s a good option for easy-to-make sandwiches, and it comes ready to eat, making it convenient even when you don’t feel like cooking. Given its ubiquity in the American kitchen, however, deli meat is also a source of uncertainty. How long does lunch meat last? Is it safe to keep past the expiration date? And why on earth do I sometimes have iridescent ham?

What causes the rainbow sheen on ham?
Rainbow sheen ham

That metallic-looking rainbow that often appears on sliced deli meat is known as iridescence, a phenomenon in which changing your position to the object causes its colors to “shimmer” and shift.

You probably also encounter this whenever you see a bit of gasoline splashed onto wet pavement. In that example, the gasoline doesn’t mix with water, but sits on top of it, forming a thin floating layer of liquid that reflects light differently from the way the surface of water does. The rainbow coloring comes from the fact that different areas of the gasoline layer have different thicknesses, and each area reflects a different band of the white light spectrum (ROYGBIV!). To the human eye, this looks like the gasoline takes on many different colors at once.