This one is kind of hard to explain without pictures... or in a timely manner. Basically when the visible light goes through the medium that splits it into the rainbow it changes angle, and part of what determines how much the angle changes is the wavelength. The reason the order and ratio is the same is because the rainbow is basically the colors ordered from longest to shortest wavelength spead a based on the change in wavelength. For instance, if you pointed a light that contained all wavelengths but yellow at a prism the rainbow would look the same as it would with all colors with a missing band where the yellow would be because the absence of yellow light does not affect the wavelengths of the other colors. That last bit is actually super important in a handful of scientific fields.