When I was waiting on a red light at an intersection in Linz I took this picture of a rainbow that was visible that day. I am always amazed how beautiful such a rainbow can be. So many different colors just from different wavelenghts of the light.
But the order of the colors that are visible is always the same.
Composition of sunlight: White sunlight contains all the colors of the visible spectrum.
Physical laws: When light enters a water droplet, it is refracted, reflected, and refracted again upon exiting. Different wavelengths: Different colors have different wavelengths. Red light is refracted less than blue or violet light.
Constant angles of refraction: Because light is refracted in every water droplet according to the same physical laws, the resulting order is always the same: red on the outside (long wavelength) and violet on the inside (short wavelength).
Rainbow at an intersection in Linz. Picture: Florian Glechner.