The refractive index is a value representing the bending of light rays as they pass from one medium to another. This value plays a critical role in understanding optics and how light behaves as it interacts with various surfaces. Willebrord Snell was in fact the first person to document the relationship between the paths taken by light rays when crossing boundaries between two surfaces and their refractive index.

Willebrord Snell was born in 1580 in a town called Leiden located in the Netherlands. Snell grew up as an astronomy and mathematics scholar, and significantly impacted both fields. He even had a lunar crater named after him. Although Snell's contributions were extensive it’s his law of refraction that played a significant role in the field of optics, and by association optometry.

In 1621 Willebrord Snell discovered a vital optical principal, now referred to as Snell's Law.  This principal, also referred to as a law, describes and details how light behaves as it passes through two connected surfaces and their respective refractive indexes as compared to to that of light in a vacuum.

The basic formula is described as follows: The ratio of the sines of the angles at which the light passes through two objects is equivalent to the phase velocities of the two media, or the reciprocal of the ratios of their respective refraction index. Below is an illustration of this formula, which is written as the following:

This law defines a phenomenon that allows for the proper understanding and predicting how light travels through different mediums. By applying the formula that defines Snell's law, one can predict certain optical results therefore providing for the ability to create and/or develop more effective optical devices, such as spectacles and telescopes.

Willebrord Snell passed away on October 30, 1626 in Leiden Netherlands, and although his life was short his discoveries proved valuable for future generations of optical specialists.