Dioptrique

This essay is known as Descartes' greatest contribution to optics, as it is the first publication of the Law of Refraction.

In the first model, he compares light to a stick that allows a blind person to discern his environment through touch.

Descartes uses a tennis ball to create a proof for the laws of reflection and refraction in his third model.

This was important because he was using real-world objects (in this case, a tennis ball) to construct mathematical theory.

In today's notation, the law of refraction states, The astronomer Jean-Baptiste Morin was noted as one of the first people to question Descartes' method in creating his theories.