Page 21 - Curriculum Visions Dynamic Book

Tinting glass
If glass is coloured (by having metal oxides added
to the melt), it will transmit some colours of white light and not others. Cobalt gives a blue tint to glass, chromium adds green, and manganese makes purple.
In traffic lights there is a white light behind each lens. The lenses are coloured (using metal oxides) to
absorb all colours except red in the red lens, amber in the amber lens, and green in the green lens. All other kinds of light are held back, absorbed, and changed to heat.
Glass will also selectively transmit waves of light beyond the range we can see. Lenses that transmit only ultraviolet or infrared light appear black to us. That is because they hold back all visible light.
Glass can also selectively hold back these invisible waves of light, allowing only visible light to get through. Heat filters in projection lamps work this way, keeping back the infrared rays so that the heat does not damage the film.
The amount of bending caused by a particular type of glass is called its refractive index. The higher the refractive index, the greater the bending. Lead glass has a much higher refractive index than soda lime glass.
Glasses are made with a wide range of refractive indexes. In a camera lens there may be seven different glasses stuck together to produce the effect the maker wants.
(Above) Some modern windshields are made of tinted glass, as shown here. Others have a shade band at the top. This localized colour band is not actually in the glass but in the plastic interlayer in the safety laminate used for windshields.
(Above) Traffic lights use coloured glass to act as filters for all parts of the spectrum except the colour they show.
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