Total Internal Reflection and Fibres

Total internal reflection (TIR) occurs when light travelling in an optically denser medium strikes a boundary with a less optically dense medium and is completely reflected back into the denser medium.

TIR occurs only when: 1. light travels from an optically denser medium to a less optically dense medium; 2. the angle of incidence is greater than the critical angle.

The **critical angle** is the angle of incidence in the denser medium for which the refracted ray travels along the boundary. At the critical angle: **angle of refraction = 90° If: i > critical angle** then total internal reflection occurs.

TIR can reflect light very efficiently. This makes it useful in: * prisms; binoculars; endoscopes; * optical fibres; * communication systems.

An optical fibre is a thin transparent strand that guides light. It normally contains: **core → higher refractive index; cladding** → lower refractive index. Light travelling in the core repeatedly undergoes total internal reflection at the core-cladding boundary.

Optical fibres can carry information as pulses of light. Advantages include: * high data capacity; * low signal loss over suitable distances; immunity to electromagnetic interference; small size.