Snell's law of refraction is a fundamental principle of optics that describes how light bends when passing from one medium to another. It is named after the Dutch mathematician Willebrord Snellius, who first formulated the law in 1621.

The law states that the angle of incidence (the angle at which light strikes a surface) and the angle of refraction (the angle at which light is bent after passing through the surface) are related to the indices of refraction of the two media. The index of refraction of a medium is a measure of how much light is bent when passing through it.

The mathematical expression for Snell's law is:

```

n1 * sin(θ1) = n2 * sin(θ2)

```

where:

* n1 is the index of refraction of the first medium

* θ1 is the angle of incidence

* n2 is the index of refraction of the second medium

* θ2 is the angle of refraction

The law can be understood by considering the following diagram:

[Image of a light ray passing from air to glass]

In this diagram, a light ray is traveling from air (n1 = 1) to glass (n2 = 1.5). The angle of incidence is θ1 and the angle of refraction is θ2. As the light ray passes from air to glass, it bends toward the normal (the line perpendicular to the surface). This is because the index of refraction of glass is greater than the index of refraction of air.

Snell's law is a powerful tool that can be used to understand a variety of optical phenomena, such as the formation of rainbows, the refraction of light through lenses, and the reflection of light from mirrors.