This rainbow is not a bitmap texture! This beautiful effect occurs in nature when lightbeams are bend by a prism of glass. Because the index of refraction varies over the wavelength of the incident light. White light (which actually is a combination of all colors) is separated into the full color spectrum.

Try to render this scene with your favorite raytracer/renderer (like POV-Ray or 3DSMAX). I guess you won't see the rainbow, because of the inverse structure of standard raytracers: Light rays are traced from the camera instead from the light.

This very smooth image was rendered by Julien Roger with 1.280.520.629 lightrays.