Page 22 - Curriculum Visions Dynamic Book

(Above) Metallic reflective sunglasses use a metal film so thin you can see through it.
(Below) If you look at the surface of a lens, you can often catch a glimpse of the surface coatings used.
Surface coating glass
Optical effects can be changed even further by coating the surface. That is done, for example, in sunglasses as well as
in camera lenses. The greatest reflectance occurs when there is a thick metal coating; then the glass simply becomes a supporting material for the metal – it is a mirror. Very thin metal coatings are put on metallic reflecting sunglasses. In this way a little light gets through even though they appear to be totally reflecting.
When light strikes a piece of glass at a glancing angle, it can be totally reflected. That makes the glass act as a mirror, which can be undesirable, especially, for example, in camera lenses. As a result, special coatings can be added that make the glass almost unreflective, allowing all light to get into the glass no matter at what angle it strikes.
Holograms
By adding small amounts of cerium oxide and copper, silver, or gold, glass can be made to behave in special ways. For example, when ultraviolet radiation is shone on it and the glass is reheated, changes in refractive index over microscopic distances can be made to occur that may be seen as strong colours. This technique can be used to produce holograms.
Photochromic glass
Most eye glasses use lead-silicate glass. To make glasses that react to light (called photochromic glasses), less than a tenth of 1% silver halide and copper is added. When light shines on the glass, the silver halide changes to metallic silver. The silver then clumps together into tiny grains. These grains are still small enough to allow people to see through the glass, but dense enough for the glass to look brown or grey. When less light falls on the glass, the chemical changes reverse, and the glass lightens again.
Mechanical properties
Because the atoms in glass are not arranged in regular patterns as they would be in a crystal, they do not fit together as well. As a result, silica glass (with no additives) is slightly less dense than a crystal of the same material (for example, quartzite). However, both calcium from lime and sodium
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