Total Internal Reflection

This lesson covers total internal reflection (TIR), including the critical angle, the conditions required for TIR, and its practical applications in optical fibres and endoscopes, as required by the Edexcel GCSE Combined Science specification (1SC0).

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What Is Total Internal Reflection?

Total internal reflection (TIR) occurs when a ray of light travelling in a denser medium hits the boundary with a less dense medium at an angle greater than the critical angle. Instead of refracting (passing through), the light is completely reflected back into the denser medium.

The Three Conditions for TIR

For total internal reflection to occur, all three of the following must be true:

1. Light must be travelling from a denser medium towards a less dense medium (e.g. glass → air, water → air).
2. The angle of incidence must be greater than the critical angle for that pair of materials.
3. The boundary must be between two transparent media.