>Did we really just make guesses and call them truth until the 1600s?
It does seem that way! There may be a Dunning-Krueger selection bias at work here - people who admitted ignorance were not remembered, while people who confidently asserted things were treated as authorities. Religious institution surely bears some blame in Western culture - notions such as "skepticism" and "seeing for yourself" were culturally suppressed for nearly 2000 years.
On the other hand, white light does contain all of the visible spectrums. Maybe Aristotle had seen prismatic diffraction and considered it beyond his powers.