What is used to colour stained-glass windows?

(Image credit: S. Hermann & F. Richter/ Pixabay)

Question from Nicholas Foster

Metal salts and tiny metal particles have coloured stained glass for thousands of years. For metal particles, this uses the electron movement that makes metals useful as wires. Electrons at metal surfaces trap some colours of light and reflect others. Most large metal sheets reflect all visible colours, so the shiny surfaces work as mirrors. Smaller metal particles trap and reflect visible colours. 

As well as beautiful stained glass, this causes strange effects. The Lycurgus Cup, a green glass goblet from 4th-century Rome held in the British Museum in London, looks red when a light is put inside it.

Answered by Andy Extance for Brain Dump in How It Works issue 133.

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