Snell's Law

Snell's Law Refraction

Around 1621, the Dutch astronomer, Willebrord Snellius, discovered a formula for the velocity of light through various materials such as glass, water, and air. An Arab fellow, named Ibn Sahl, discovered the law about 600 years earlier. The formula shows that light slows down according to the density of the matter it penetrates.

Formula for Snells Law

After Snellius died, others claimed they had discovered it too:

Portrait of Willebrord Snellius