Willebrord Snellius was a Dutch astronomer and mathematician.

In the west, especially the English speaking countries, his name has been attached to the law of refraction of light for several centuries, but it is now known that this law was discovered by Ibn Sahl in 984.

The same law was also investigated by Ptolemy and in the Middle Ages by Witelo, but due to lack of adequate mathematical instruments (trigonometric functions) their results were saved as tables, not functions.

To add to the confusion,

(1)  It is also sometimes referred to as the Snell-Descartes law. 

(2)  Snell's law follows from Fermat's principle of least time

(3)  Which in turn follows from the propagation of light as waves.