Snell's law: light will be reflected and refracted when it hits the interface of different media. The incident light and refracted light are in the same plane, and the included angle with the interface normal satisfies the following relationship:

n 1sinθ 1 = n2sinθ2

Where n 1 and n2 are the refractive indices of the two media respectively, and θ 1 and θ2 are the angles between the incident light (or refracted light) and the normal of the interface respectively, which are called incident angle and refraction angle.

The above formula is also called snell formula.

According to the formula, only sina/sinb=va/vb is deduced.

As we all know, it takes the shortest time for light to travel from one point to another.