1. Refraction of waves (and of light)

2. Refraction of wavefronts
As the wavefronts move through the medium, they remain moving at a constant speed.
When they reach a boundary between media, the speed changes
When the speed changes, the wavelength changes
What becomes visible is an obvious change in the direction in which the wavefront is moving

3. Refraction of rays
If, instead of using wavefronts, we show the direction change due to the wavespeed change using rays, we get 

4. Refraction of rays
We can quantify the change in the speed by looking at the angles.
The bigger the difference in the angles the rays make relative to the normal line, the larger the speed change has been.

5. 𝒏 𝒊 𝐬𝐢𝐧 𝜽 𝒊 = 𝒏 𝒓 𝐬𝐢𝐧 𝜽 𝒓 OR 𝒏 𝟏 𝐬𝐢𝐧 𝜽 𝟏 = 𝒏 𝟐 𝐬𝐢𝐧 𝜽 𝟐
Refraction of Light
Snell’s Law:
At a boundary between two media, the ratio of the sines of the angles made by a wave with respect to the normal line will be a constant.
Constant  related to the Refractive Index (or, index of refraction), a property of the medium itself
𝒏 𝒊 𝐬𝐢𝐧 𝜽 𝒊 = 𝒏 𝒓 𝐬𝐢𝐧 𝜽 𝒓 OR 𝒏 𝟏 𝐬𝐢𝐧 𝜽 𝟏 = 𝒏 𝟐 𝐬𝐢𝐧 𝜽 𝟐

6. Also: the Refractive Index for a medium is a ratio of the speed of light IN THAT MEDIUM to the speed of light in a vacuum. (c)
c = speed of light in a vacuum = 3.00 x 108 m·s-1
and
v = speed of light in the medium