Total Internal Reflection (Also seen in GCSE Physics 3)

We know that whenever light travels from one material to another the majority of the light refracts but a small proportion of the light also reflects off the boundary and stays in the first material.

When the incident ray strikes the boundary at an angle less than the critical angle the light refracts into the second material.

When the incident ray strikes the boundary at an angle equal to the critical angle all the light is sent along the boundary between the two materials.

When the incident ray strikes the boundary at an angle greater than the critical angle all the light is reflected and none refracts, we say it is total internal reflection has occurred.