LAWS OF REFRACTION: There are  two laws that define the behavior of light during refraction. 

First law - Snell’s law: Snell's law is a formula, used to describe the relationship between the angle of incidence and the angle of refraction, when referring to light, passing through a boundary between two different isotropic media, such as water, glass, or air.

In optics, the law is used, in ray tracing, to compute the angles of incidence or refraction, and,  in experimental optics,  to find the refractive index of a material. 

It states that the ratio between the sine of the angle of incidence, and the sine of the angle of refraction for two given optical media is a constant.
Sine, 1 upon Sine, r is constant

Snell's law also states that, the ratio of the sines of the angles of incidence and refraction is equivalent to the ratio of phase velocities in the two media, or equivalent to the reciprocal of the ratio of the indices,  of refraction as shown below

Sine , sine theta 2 by sine  sine theta 1 is equal to nu 2 by nu 1 is equal to n 1 by n 2. 

Where each 𝜃 is the angle measured from the normal of the boundary, nu is the velocity of light in the respective medium 

Second law: The incident ray, the refracted ray, and the normal at the point of incidence, lies in the same plane as shown in the diagram above.

Refractive Index (or n):

1) Absolute refractive Index: Refractive index, is also called an index of refraction. It measures the bending of a ray of light,  when passing from one medium into another. 

If i is the angle of incidence of a ray in a vacuum,  and r is the angle of refraction, the refractive index 𝜇 is defined, as the ratio of the sine of the angle of incidence to the sine of the angle of refraction; that is  𝜇 = sine i / sine r. 

Refractive index is also equal to the velocity of light c, of a given wavelength, in empty space divided by its velocity v in a substance, or 𝜇 = c by v.

Thus in short, it is defined as the ratio of Speed of light in free space (c) to that of the speed of light in a given medium (v).
Therefore Mu is equal to c by V

It has no units and no dimensional formula.

For a given light, denser the medium,  lesser will be the speed of light and so greater will be the refractive Index. Thus the velocity of light in glass,  is less than the velocity of light in water as shown below.
v glass is less than v water . Mu G is less than Mu W

Consider the figure given below

For a given light and a medium, the refractive index is equal to the ratio of the wavelength of light in free space to that in a medium.
Mu is equal to lamda zero by lamda

𝜆0 = wavelength in free space
𝜆 = wavelength in medium

NGSS

Common Core State Standards Connections: