Total Internal Reflection

This lesson covers total internal reflection (TIR) as required by the Edexcel GCSE Physics specification (1PH0), Topic 5: Light and the Electromagnetic Spectrum. You need to understand the conditions for TIR, the concept of the critical angle, and the practical applications of TIR including optical fibres.

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What Is Total Internal Reflection?

Total internal reflection (TIR) occurs when a ray of light travelling inside a denser medium (e.g. glass or water) hits the boundary with a less dense medium (e.g. air) and is completely reflected back inside the denser medium. No light passes through the boundary — all of it is reflected.

This is different from normal reflection because in normal circumstances some light would pass through (refract) and some would reflect (partial reflection). In TIR, 100% of the light is reflected.

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The Critical Angle

The critical angle is the angle of incidence above which total internal reflection occurs.

What Happens at Different Angles