The Kopp–Etchells Effect

I’d never heard of it either. But now I have.

It’s the light produced when dust or sand particles strike the abrasive hard edge of a helicopter rotor blade: an example of a pyrophoric effect. And it’s quite spectacular:

Like sparks from a grindstone:

When a speck of metal is chipped off the rotor, it is heated by rapid oxidation. This occurs because its freshly exposed surface reacts with oxygen to produce heat. If the particle is sufficiently small, then its mass is small compared to its surface area, and so heat is generated faster than it can be dissipated. This causes the particle to become so hot that it reaches its ignition temperature. At that point, the metal continues to burn freely.

Add a dark background and some long exposure, and you can get some amazing images:

Named for two soldiers killed in Afghanistan, the phenomenon might be nice to look at, but can cause problems in the field. Erosion of the rotor blades is a serious issue, and the light produced can not only affect pilots’ night vision, but can also alert the enemy to the exact location of a helicopter or base.