Total Internal Reflection and Lenses

This lesson covers total internal reflection and the properties of converging and diverging lenses — as required by the Edexcel GCSE Physics specification (1PH0), Topic 4: Waves. You need to understand the conditions for total internal reflection, its applications, and how to draw ray diagrams for lenses. Some content in this lesson is Higher Tier only and is clearly marked.

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Total Internal Reflection

Total internal reflection (TIR) occurs when light travelling through a denser medium (e.g. glass or water) hits the boundary with a less dense medium (e.g. air) at an angle greater than the critical angle. Instead of being refracted out of the denser medium, all the light is reflected back inside.

Conditions for Total Internal Reflection

For total internal reflection to occur, both of these conditions must be met:

1. Light must be travelling from a denser medium towards a less dense medium (e.g. glass → air).
2. The angle of incidence must be greater than the critical angle for that boundary.

What Is the Critical Angle?