The figures below show three rays of light incident from water to air at different angle of incidence. The light moves from a dense to a less dense medium, so it bends away from the normal.

N.B.: As light moves from water to air, it is completely reflected when the angle of incidence is greater than a certain value.

When the angle of incidence is equal to C (critical angle), the angle of refraction is equal to 90^o.

When the angle of incidence is greater than C, then there is no refraction; rather the ray is completely reflected into the water medium and this is called total internal reflection.

N.B; The critical angle is defined as the angle of incidence which gives an angle of refraction of 90^o when light is moving from a dense to a less dense medium.

Therefore for total internal reflection to occur;

1. The light must be travelling from a dense to a less dense medium. Total internal reflection can occur when light moves from glass to water but not from water to glass and vice versa
2. The angle of incidence must be greater than the critical angle. For light moving from a medium into air, the critical angle is given by;

                            n= 1sinC or 1n=s��n C

Therefore, critical angle usually refers to medium-air boundaries.

For example glass has critical angle of about 42^o which means that for light moving from glass to air, the angle of refraction is 90^o if the angle of incidence is 42^o. N.B; For total internal reflection i=C and r=90^o.

Therefore, for light moving from glass to air, total internal reflection can only occur if the angle of incidence is greater than 42^o.