When it comes to total internal reflection and refraction then you'd do well to remember Snell's Law

$\displaystyle n_1 \sin\theta _i = n_2 \sin\theta _r$ where:

- $\displaystyle n_1$ = Refractive index of the incident material (the material it's leaving)
- $\displaystyle n_2$ = Refractive index of the material it's entering
- $\displaystyle \theta _i$ = incident angle
- $\displaystyle \theta _r$ = refracted angle

The critical angle is when $\displaystyle \theta _r = \frac{\pi}{2} = 90^{\circ}$

$\displaystyle \theta _c = \arcsin \left(\dfrac{n_2}{n_1}\right)$

For incident values above this total internal reflection occurs (and it's always the same angle as the incident so you draw a perpendicular and sketch the same angle)