What does the rainbow effect indicate when observed on a surface?

The rainbow effect observed on a surface indicates a diffraction pattern from hydrocarbons. This effect occurs when light interacts with a thin layer of hydrocarbons or other similar substances. When light passes through or reflects off these layers, it can bend and separate into different wavelengths, producing a spectrum of colors reminiscent of a rainbow.

This phenomenon is particularly evident when oil or gasoline is present on a wet surface, such as pavement after rain. The light interference patterns created by the oil layer result in vivid color displays, which are characteristic of the diffraction process at a molecular level. Understanding this effect is crucial for fire investigators as it can help them determine the presence of flammable liquids and contribute to the analysis of the scene.