Total Internal Reflection and Fibres

Total Internal Reflection and Fibres

Light can sometimes be trapped inside glass and keep reflecting instead of leaving.

This is called total internal reflection.

Optical fibres use this effect to guide light.

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.

Two Conditions

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.

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.

Why TIR Is Useful

TIR can reflect light very efficiently.

This makes it useful in:

prisms; binoculars; endoscopes; optical fibres; * communication systems.

Optical Fibre

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.

Communication

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.

Total Internal Reflection and Fibres — Advanced

TIR follows from Snell's law when refraction into the lower-index medium is no longer possible for incidence angles above the critical angle.

For indices `n₁ > n₂`:

**sin c = n₂n₁**

where `c` is the critical angle.