Question

Who invented the law of refraction?

Answer 1

This is quite a good question because by researching it I discovered a new thing (or at least that is new for me)!!! The person that first described the law of refraction is Ibn Sahl (984).

Explanation:

Normally we call it "Snell's Law" from the Dutch mathematician Willebrord Snellius (1580-1626). But according to Wikipedia it was first discovered by Ibn Sahl (Arabic: ابن سهل‎) (Abu Saʿd al-ʿAlaʾ ibn Sahl ) (Arabic: أبو سعد العلاء ابن سهل‎) (c. 940–1000) who was a Muslim mathematician, physicist and optics engineer of the Islamic Golden Age associated with the Abbasid court of Baghdad.
Although I do not like to cite Wikipedia I had a look to the reference:

Thomas Hockey et al. (eds.). The Biographical Encyclopedia of Astronomers, Springer Reference. New York: Springer, 2007, p. 567.

and it seems right although: "... he states a geometrical relation between incident and refracted rays that, rewritten in modern trigonometric notation, is equivalent to the Law of Refraction, although it does not involve the notion of the refractive index of a medium."

There is also an extract from his manuscript

[Reproduction of a page of Ibn Sahl's manuscript showing his discovery of the law of refraction (from Rashed, 1990)]

If you want an interpretation of the above picture have a look at:
https://spie.org/etop/2007/etop07fundamentalsII.pdf