Snell's Law: n1*sin(theta1) = n2*sin(theta2)
N is the index of refraction of the medium, theta is the angle of the light ray in the medium. This was used to calculate the surfaces of "burning lenses."
Lots of algebra was invented by Muslims. The word itself is Arabic. So were the principles of navigation that allowed Europeans to conquer the world once they recovered from the dark ages. Azimuth, for instance, is Arabic.
They weren't always as they are now.