Page 55 - Curriculum Visions Dynamic Book

Optical glass fibres
The most significant development in glass-fibre manufacture in recent years has been making optical- quality glass in fibre form.
Glass fibres are used for sending
digital telecommunications. A laser light is
used as a source. As the light moves down the fibre, it bounces against the sides of the fibre. However, because it always hits the sides of the glass at a low angle, the light is always reflected back into the glass, and it never escapes, even when the glass is bent.
Optical fibres, including their protective glass sheathing, are just over the width of a hair, some 125 microns (millionths of a metre) across, with the transmission fibre being about 10 microns across.
The first glass fibres were made of glass that contained impurities, and they had very limited use. But by the 1960s the glass was of sufficient purity
to allow infrared light to be used as a source of information over long distances. Some plastic fibres are also used as optical fibres for short lengths of fibre, but they do not have the high reflecting properties of glass and absorb the light quite quickly.
(Above) Glass fibre keeps light inside because the light always strikes the side of the glass at more than the critical angle and is always reflected back inside the glass.
The actual light-carrying fibre is enclosed in a sheath of glass with a different reflectivity to ensure no light gets out.
(Below) Light passing through optical fibres.
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