Snell’s Law, named after Willebrord van Roijen Snell, is the main law concerned with which specific physical phenomenon?

Snell’s law is a relationship between the path taken by a ray of light in crossing the surface of separation between one medium and another and the density of each. This law was discovered in 1621 by Dutch astronomer and mathematician Willebrord Snell (also called Snellius).

When light is travelling straight down from one medium to another e.g. air to water or vice versa, then refraction doesn't occur, even if the surface is curved. It happens when a light ray approaching a surface from any angle other than the perpendicular and one medium is optically denser than the other. How much it bends depends on the difference in optical density and the wavelength (different wavelengths refract differently, red bending least and violet the most). If it moves to a denser medium e.g. air to water, it bends towards the line of the perpendicular. If it moves from a denser medium e.g. water to air, it bends away from the perpendicular.

The refraction phenomenon was the subject of interest in much earlier periods of history. Although named after Dutch astronomer Willebrord Snellius (1580–1626), the relationship of refraction angles was first conceptually examined by Ptolemy (circa AD 100 - 170), later by the Persian scientist Ibn Sahl in AD 984. In 1602 it was "rediscovered" by Thomas Harriot (who didn't publish his findings) and then by Snell. The account of Snell’s law went unpublished until its mention by Christiaan Huygens in his treatise on light “Traité de la Lumière” in1678.