Optical fibres

An optical fibre is a thin strand of high-quality glass. Light can be transmitted through it over very large distances.

The optical fibre makes use of the 'total internal reflection' of light.

Total internal reflection and the critical angle

When a ray of light leaves a denser medium, it moves away from the normal.

Light travels through a block of glass at an angle to the perpendicular, and exits into the air at a greater angle.

As can be seen from the above diagram, as the angle of incidence inside the material increases, the angle of refraction increases, to a point at which it leaves the material at 90˚.

As the angle of the light inside the block increases relative to the perpendicular, it exits parallel to the edge of the glass.

The minimum angle at which total internal reflection occurs is known as the critical angle (for Perspex this is about 43˚). At any angle greater than the critical angle, total internal reflection occurs and the light ray obeys the normal rules for reflection (ie angle of incidence = angle of reflection).

If the angle of incidence is increased any more, then the ray does not exit glass into the air, but is reflected inside the glass. As none of the light refracts into the air, the ray has been totally internally reflected inside the material.

As the angle increases further, the light is reflected back inside the glass, rather than leaving it.

Very little light is absorbed in the glass. Light travels from one end to the other of an optical fibre by total internal reflection, even when the fibre is bent.

A cross-section of an optical glass fibre with a ray of light bouncing off the curved glass, labelled 'total internal reflection'.

Optical fibres are used in telecommunications because they can carry enormous amounts of information in light pulses trapped inside them. This information is carried at very high speed (approximately 2 \times 10^{8}m\,s^{-1}) along an optical fibre cable.

Fibre optic communication can transmit much more data at once than electrical cables.