Hellins law, also known as "Snell's Law" or "the law of refraction", is a fundamental principle in optics that describes how light waves behave when they pass through different media. This law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is always constant for a given pair of media. Synonymous with "Hellins law", the term "Snell's law" is derived from the Dutch mathematician Willebrord Snell, who first formulated this principle in the 17th century. Both terms can be used interchangeably to describe this law in the field of optics, where it is relied upon for understanding and predicting the behavior of light waves.