What does the "rainbow effect" refer to in fire investigations?

The "rainbow effect" in fire investigations specifically refers to the diffraction pattern that occurs when hydrocarbons, such as oil or gasoline, float on a surface of water. This phenomenon creates a colorful display resembling a rainbow due to the way that light interacts with the thin film of hydrocarbons. When light hits this film, it can bend and separate into its component colors, producing a spectrum that is visibly similar to a rainbow. This effect can be critical for investigators as it can indicate the presence of flammable liquids at a scene and assist in determining the origin and cause of a fire. Understanding this phenomenon helps fire investigators recognize potential evidence that may explain the conditions leading to the fire incident.