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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 bring the ray out 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 ray changes from just refracting to TIR is called the critical angle.

Uses of total internal reflection (TIR)

Total internal reflection allows light to be contained and guided along very thin fibres. Usually made of glass, these are called optical fibres and they have many uses:

• fibre broadband internet sends computer information coded as pulses of light along underground optical fibres
• doctors can look at the inside of their patients using an endoscope - a long tube which guides light into the patient and then guides the reflected light back out to give an image
• decorations, like some artificial Christmas trees, carry coloured light to different parts of the decoration and let it shine out in different directions

Optical fibre construction - Higher

Different types of glass have slightly different densities. This causes slight variations in the speed when light travels through the different types of glass. By making an optical fibre’s structure from different types of glass, the light can be made to refract slightly as it travels along the fibre. This can control the timing of light pulses in the fibre and improves the quality of signal transmissions.