Snell is credited with the laws of reflection and refraction. However, Ibn Al-Haitham discovered the same phenomena in the 11th century.
He was arguably the greatest Muslim physicist and one of the greatest students of optics of all time. He was also an astronomer, a mathematician, a physician, and he wrote commentaries on Aristotle and Galen. He wrote about 70 manuscripts and he propounded what we now call Snell's law about 600 years before Snell.
The Latin translation of his main work, the Optics (Kitab al-manazir), exerted a great influence upon Western science (Roger Bacon; Kepler). It showed great progress in the experimental method in the following areas: research in catoptrics: spherical and parabolic mirrors, spherical aberration; in dioptrics: the ratio between the angle and incidence and refraction does not remain constant; the magnifying power of a lens; study of atmospheric refraction - twilight only c eases or begins when the sun is 19º below the horizon; an attempt to measure the height of the atmosphere on that basis; better description of the eye, and better understanding of vision, although Ibn al-Haitham considered the lens as the sensitive part; the rays originate in the object seen, not in the eye; an attempt to explain binocular vision; a correct explanation of the apparent increase in the size the sun and the moon when near the horizon; the earliest use of the camera obscura.
Catoptrics contains the following problem, known as Alhazen's problem: from two points of the plane of a circle to draw lines meeting at point of the circumference and making equal angles with the normal at that point. It leads to an equation of the fourth degree. Alhazen solved it with the aid of a hyperbola intersecting a circle. He also solved the so-called al-Mahani's (cubic) equation (q. v., second half of the ninth century) in a similar (Archimedean) manner.