>Rainbows aren't possible without a firmament
Explain how you get rainbows in your sprinkler then.
The reason it's not easy to make rainbows with indoor lighting is that the lights don't contain the full visible spectrum. Water droplets serve the exact same purpose as a prism, having a different refractive index from the atmosphere surrounding it, the different wavelengths of light will refract at different angles, spreading the different wavelengths out.