Total internal reflection is an optical phenomenon that happens when a ray of light strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary, no light can pass through and all of the light is reflected. The critical angle is the angle of incidence above which the total internal reflection occurs.
The larger the angle to the normal, the smaller is the fraction of light transmitted, until the angle when total internal reflection (blue line) occurs. (The color of the rays is to help distinguish the rays, and is not meant to indicate any color dependence.)
The equation for the critical angle:θC = arcsin (n2/n1)