Page 56 - Curriculum Visions Dynamic Book

More and more lens glasses are, however, being made of plastic because of weight. The light bending you can achieve per unit weight is important to the wearer.
7. Telescope lenses and mirrors
Telescopes lenses require the highest grade of optical glass. Instead of making a large telescope lens out of a single piece of glass as was done in the past, modern telescope optics are made from small balls of high-purity glass that are fused together to make the disk the lens is ground from.
Even mirrors (which use a silver coating on glass) need high-precision glass. The largest telescope glass in the world (at Mauna Kea, Hawaii) is 8.31 metres in diameter, 22cm thick, and weighs 33 tonnes. It was made from very low expansion glass in fused pieces and then ground to the right shape before having a surface coating added.
8. Optical fibres
Glass many times thinner than a human hair is used to carry signals at the speed of light. This is the fastest, most reliable, and most modern way of transmitting information in the digital age. Most long-distance phone, TV, and computer data lines use optical fibres.
Beams of light
For long fibres to work, information is sent in the form of pulses of light through a fibre core. The core is sheathed in glass that has a lower refractive index than the core – the glass is made so that its properties change from
the centre to the outside. This arrangement
ensures that as the light tries to leak out of
the core, the light rays are bounced back
into the core. In effect, the outer sheath
of glass acts like a mirror. A single beam
of light needs a fibre just ten millionths
of a metre (microns) in diameter.
Multiple beams can be sent in cores of
diameter 50 microns or larger. Multiple
beams (called multimode) are used for short-
distance communication, with simple beams (monomode) used for long-distance communications, with many signals pulsed down the same simple beam at the same time.
High-index glass
Low-index glass
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