Which chemicals make a rainbow?

Answer 1

A rainbow is formed as a full visible spectrum, where there is a completely graded system of particle size, or of viewing angle. The common airborne rainbow is composed of water droplets, which, depending on the observers angle, span the full visible spectrum. You may also see a rainbow of colour on the surface of a CD, where the grooves form a diffraction grating. Another different rainbow is formed when a thin film of one liquid is spread out over another fluid - without mixing. This is your common gasoline rainbow on a puddle. Back in the day when film transparencies were commonly mounted in glass covers, then 'Newton's Rings' of a rainbow may show as the angle between the media change. And of course, a rainbow is formed when white light passes through a prism - the chandelier and the diamond ring are examples.

So, the phenomenon is one of angle, not particularly of chemical nature.