Incident Angle (θ)Angle from the normal to the surface.
Refraction Index (n)

n = c / v = λair / λmedium

Refraction index of air: 1
Refraction index of water: 1.33

RefractionFor a ray travelling from one medium into another,

n1 sin θ1 = n2 sin θ2

1 refers to incident side; 2 refers to refracting side
n = Index of refraction of medium
θ = Incident angle to medium

Sometimes when solving for θ1 or θ2, it cannot be defined.
For example, if θ1 = sin-1 [ ( n2 sin θ2 ) / n1 ] and is greater than 1.
Secant of any value greater than one is impossible.
In this case, it means there is total internal reflection and the ray never exits the medium.
You can take the θ1 to be 90° in this case.

Critical Angle = θc

The angle θc is where total internal reflection (TIR) occurs, whereby all light entering from medium1 to medium2 is reflected back into medium1.

n1 sin θc = n2 sin 90°
n1 sin θc = n2
θc = sin-1 ( n2 / n1 )