Critical angle and total internal reflection

This animation shows how light behaves when it travels from glass to air. When the angle of incidence in glass is <42^o most of the light refracts and leaves the glass. However when the angle of incidence is 42^o or more ALL the light is reflected back inside the glass. This is called total internal reflection and the angle at which this happens the critical angle for the glass.

Notice that when the angle of incidence is less than the critical angle some of the light is reflected back inside the glass.