I think therefore I am
In Descartes' scheme, a light ray (here coming from upper left) hits a raindrop, penetrates it, is reflected internally one or more times, and finally exits. When a light ray moves from air to water, it changes direction. We now know - at least in some sense - that this is because the light travels in waves, and it possesses a slower speed inside water.
Descartes law of refraction
Rene Descartes maps it out
a French philosopher, mathematician, and scientist. Dubbed the father of modern western philosophy, much of subsequent Western philosophy is a response to his writings, which are studied closely to this day. He spent about 20 years of his life in the Dutch Republic.!