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White created by light - on a television screen, say, or through a prism - has no wavelength of its own. Instead, it is the sum of all the colours. Think of a rainbow: you cannot see the colours hidden in sunlight until raindrops bend the light and reveal the whole spectrum. For this reason, scientists do not usually…

• Why does the writer say that white is not scientifically a colour?
• What material does the writer say was used before recorded history?