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When a light ray reaches the boundary between two transparent materials it may be refracted.

If it is leaving the more dense medium, this refraction would be expected to bend the ray away from the normal as it emerges.

However, if this would bend the ray at more than 90° from the normal, the refraction is not possible.

In this situation, the ray is reflected inside the more dense medium, following the law of reflection.

This is called total internal reflection (TIR).

The angle of incidence when the angle of refraction is 90^o, and the ray changes from just refracting to total internal reflection, is called the critical angle.

Total internal reflection only occurs when:

• light travels from a dense medium to a less dense medium e.g. from glass to air;
• the angle of incidence in the dense medium is greater than the critical angle;
• if the angle of refraction in the air becomes 90°, the angle of incidence in the glass is called the critical angle;
• if the angle of incidence in the glass is greater than the critical angle, total internal reflection occurs.

The diagram below shows the light refracting from glass into air.

For light travelling from glass into air the angle of refraction is greater than the angle of incidence.

When the angle of refraction is exactly 90°, then the angle of incidence is called the critical angle C.

When the angle of incidence is greater than the critical angle none of it is refracted, the ray is totally internally reflected, and the law of reflection is obeyed, i = r.